diff --git a/viewer/CMakeLists.txt b/viewer/CMakeLists.txt
index eaf9d1ddb..0531d5062 100644
--- a/viewer/CMakeLists.txt
+++ b/viewer/CMakeLists.txt
@@ -25,6 +25,8 @@ ADD_EXECUTABLE(jfjoch_viewer jfjoch_viewer.cpp JFJochViewerWindow.cpp JFJochView
         JFJochViewerMenu.h
         RemoteDisplayMode.cpp
         RemoteDisplayMode.h
+        ViewerTheme.cpp
+        ViewerTheme.h
         image_viewer/JFJochDiffractionImage.cpp
         image_viewer/JFJochDiffractionImage.h
         image_viewer/JFJochSimpleImage.cpp
diff --git a/viewer/JFJochViewerMenu.cpp b/viewer/JFJochViewerMenu.cpp
index 60f5cf129..1c5d06305 100644
--- a/viewer/JFJochViewerMenu.cpp
+++ b/viewer/JFJochViewerMenu.cpp
@@ -3,6 +3,7 @@
 
 #include "JFJochViewerMenu.h"
 #include "RemoteDisplayMode.h"
+#include "ViewerTheme.h"
 
 #include <QMessageBox>
 #include <QApplication>
@@ -90,6 +91,22 @@ JFJochViewerMenu::JFJochViewerMenu(QWidget *parent) : QMenuBar(parent) {
     remoteMode->setChecked(RemoteDisplayMode());
     connect(remoteMode, &QAction::toggled, this, [](bool on) { SetRemoteDisplayMode(on); });
 
+    viewMenu->addSeparator();
+    QMenu *themeMenu = viewMenu->addMenu("Theme");
+    auto *themeGroup = new QActionGroup(this);
+    const ThemeChoice currentTheme = LoadThemeChoice();
+    const std::pair<ThemeChoice, QString> themes[] = {{ThemeChoice::System, "Follow system"},
+                                                      {ThemeChoice::Light, "Light"},
+                                                      {ThemeChoice::Dark, "Dark"}};
+    for (const auto &theme: themes) {
+        const ThemeChoice choice = theme.first;
+        QAction *action = themeMenu->addAction(theme.second);
+        action->setCheckable(true);
+        action->setChecked(choice == currentTheme);
+        themeGroup->addAction(action);
+        connect(action, &QAction::triggered, this, [choice] { SetThemeChoice(choice); });
+    }
+
     viewMenu->addSeparator();
     // The desktop's own text scaling stays the primary mechanism, but few users know where it lives,
     // so the viewer carries a zoom of its own. It multiplies the font the desktop handed us, so it
diff --git a/viewer/JFJochViewerStatusBar.cpp b/viewer/JFJochViewerStatusBar.cpp
index 64ba1ed4b..f8c350ee2 100644
--- a/viewer/JFJochViewerStatusBar.cpp
+++ b/viewer/JFJochViewerStatusBar.cpp
@@ -18,6 +18,8 @@ JFJochViewerStatusBar::JFJochViewerStatusBar(QWidget *parent) : QStatusBar(paren
     state_box->setTextVisible(true);
     state_box->setAlignment(Qt::AlignCenter);
     state_box->setFormat("");
+    // No detector session yet: the badge keeps its place but draws nothing (see setHttpConnection).
+    state_box->setStyleSheet("QProgressBar { border: none; background: transparent; }");
     addPermanentWidget(state_box);
 }
 
@@ -58,7 +60,7 @@ void JFJochViewerStatusBar::setHttpConnection(bool connected, QString addr) {
         // No detector session at all -> blank the whole cluster (but keep the reserved space).
         state_box->setFormat("");
         state_box->setValue(0);
-        state_box->setStyleSheet("");
+        state_box->setStyleSheet("QProgressBar { border: none; background: transparent; }");
         state_box->setToolTip("");
         rate_label->setText("");
         return;
diff --git a/viewer/ViewerTheme.cpp b/viewer/ViewerTheme.cpp
new file mode 100644
index 000000000..02361d54c
--- /dev/null
+++ b/viewer/ViewerTheme.cpp
@@ -0,0 +1,150 @@
+// SPDX-FileCopyrightText: 2026 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
+// SPDX-License-Identifier: GPL-3.0-only
+
+#include "ViewerTheme.h"
+
+#include <QApplication>
+#include <QChart>
+#include <QPalette>
+#include <QSettings>
+#include <QStyle>
+#include <QStyleHints>
+#include <QWidget>
+
+namespace {
+    bool dark_theme = false;
+
+    const QColor DARK_WINDOW(0x1A, 0x1D, 0x3A);   // a deep indigo
+    const QColor DARK_TEXT(0xE3, 0xE7, 0xEC);
+
+    bool SystemIsDark() {
+#if QT_VERSION >= QT_VERSION_CHECK(6, 5, 0)
+        return QGuiApplication::styleHints()->colorScheme() == Qt::ColorScheme::Dark;
+#else
+        return false;
+#endif
+    }
+
+    // Text roles are set for the active and inactive groups only, so disabled text keeps the greyed
+    // colour the palette derived for it.
+    void SetTextColor(QPalette &pal, const QColor &color) {
+        for (auto group: {QPalette::Active, QPalette::Inactive})
+            for (auto role: {QPalette::WindowText, QPalette::Text, QPalette::ButtonText, QPalette::ToolTipText})
+                pal.setColor(group, role, color);
+    }
+
+    // Salmon panels, white entry fields that fall back to salmon when disabled, and a navy accent for
+    // selections and progress-bar chunks. It starts from Fusion's own (light) palette, not the system
+    // one, which is dark under a dark OS theme; the text is set to black because on macOS the palette's
+    // text colours come from the platform theme and are grey.
+    QPalette LightPalette() {
+        QPalette pal = QApplication::style()->standardPalette();
+        pal.setColor(QPalette::Window, QColor(255, 235, 230));
+        pal.setColor(QPalette::Base, Qt::white);
+        pal.setColor(QPalette::AlternateBase, QColor(255, 245, 242));
+        pal.setColor(QPalette::Button, QColor(255, 235, 230));
+        pal.setColor(QPalette::Disabled, QPalette::Base, QColor(255, 235, 230));
+        pal.setColor(QPalette::Highlight, QColor(0x1f, 0x3a, 0x5f));
+        pal.setColor(QPalette::HighlightedText, Qt::white);
+        SetTextColor(pal, Qt::black);
+        return pal;
+    }
+
+    // The same layout in the dark: indigo panels, entry fields darker still, and a lighter navy for
+    // selections so they stand out from the panels. QPalette(button) derives the bevel shades and the
+    // disabled text from the one colour.
+    QPalette DarkPalette() {
+        QPalette pal(DARK_WINDOW);
+        pal.setColor(QPalette::Window, DARK_WINDOW);
+        pal.setColor(QPalette::Base, QColor(0x12, 0x14, 0x2B));
+        pal.setColor(QPalette::AlternateBase, QColor(0x24, 0x28, 0x4A));
+        pal.setColor(QPalette::Button, DARK_WINDOW);
+        pal.setColor(QPalette::Disabled, QPalette::Base, DARK_WINDOW);
+        pal.setColor(QPalette::ToolTipBase, QColor(0x2A, 0x2E, 0x55));
+        pal.setColor(QPalette::Highlight, QColor(0x2F, 0x5A, 0x8F));
+        pal.setColor(QPalette::HighlightedText, Qt::white);
+        pal.setColor(QPalette::Link, QColor(0x8A, 0xB4, 0xF8));
+        pal.setColor(QPalette::PlaceholderText, QColor(0x80, 0x84, 0xA8));
+        SetTextColor(pal, DARK_TEXT);
+        return pal;
+    }
+
+    void ApplyTheme() {
+        const ThemeChoice choice = LoadThemeChoice();
+        dark_theme = choice == ThemeChoice::Dark || (choice == ThemeChoice::System && SystemIsDark());
+        QApplication::setPalette(dark_theme ? DarkPalette() : LightPalette());
+        // With an application stylesheet in place, widgets take their palette when they are polished
+        // and do not see a later palette change; setting the stylesheet again re-polishes them all.
+        // Fusion fills QGroupBox interiors with a flat colour of its own; transparent makes them show
+        // the window background like the rest of the UI (the dock resize handles are drawn by
+        // SeparatorHandleStyle, not the stylesheet).
+        qApp->setStyleSheet("QGroupBox { background-color: transparent; }");
+        emit ThemeNotifier::Instance()->changed();
+    }
+}
+
+ThemeNotifier *ThemeNotifier::Instance() {
+    static ThemeNotifier notifier;
+    return &notifier;
+}
+
+ThemeChoice LoadThemeChoice() {
+    const QString value = QSettings("PSI", "jfjoch_viewer").value("theme", "system").toString();
+    if (value == "light") return ThemeChoice::Light;
+    if (value == "dark") return ThemeChoice::Dark;
+    return ThemeChoice::System;
+}
+
+void SetThemeChoice(ThemeChoice choice) {
+    const char *value = "system";
+    if (choice == ThemeChoice::Light) value = "light";
+    if (choice == ThemeChoice::Dark) value = "dark";
+    QSettings("PSI", "jfjoch_viewer").setValue("theme", value);
+    ApplyTheme();
+}
+
+void InitTheme() {
+    ApplyTheme();
+#if QT_VERSION >= QT_VERSION_CHECK(6, 5, 0)
+    // Re-applied on any desktop scheme change, not only under System: the palette set here must also
+    // survive the platform reacting to the change with a palette of its own.
+    QObject::connect(QGuiApplication::styleHints(), &QStyleHints::colorSchemeChanged,
+                     ThemeNotifier::Instance(), [] { ApplyTheme(); });
+#endif
+}
+
+bool DarkTheme() {
+    return dark_theme;
+}
+
+void SetThemedStyleSheet(QWidget *widget, const std::function<QString()> &style) {
+    widget->setStyleSheet(style());
+    QObject::connect(ThemeNotifier::Instance(), &ThemeNotifier::changed, widget,
+                     [widget, style] { widget->setStyleSheet(style()); });
+}
+
+QColor AccentTextColor() {
+    return dark_theme ? QColor(0x9D, 0xBB, 0xE3) : QColor(0x1F, 0x3A, 0x5F);
+}
+
+QColor IconInkColor() {
+    return dark_theme ? DARK_TEXT : QColor(0x2B, 0x2B, 0x2B);
+}
+
+QString SliderStyle() {
+    // On the dark panels the navy handle would vanish, so it takes the light blue of the headings.
+    const QString groove = dark_theme ? "#363A66" : "#F3D9D4";
+    const QString handle = dark_theme ? "#9DBBE3" : "#1F3A5F";
+    const QString handle_hover = dark_theme ? "#BCD2EE" : "#16314F";
+    return QString(
+        "QSlider::groove:horizontal { height:6px; background:%1; border-radius:3px; }"
+        "QSlider::sub-page:horizontal { background:#FA7268; border-radius:3px; }"
+        "QSlider::handle:horizontal { background:%2; width:14px; margin:-5px 0; border-radius:7px; }"
+        "QSlider::handle:horizontal:hover { background:%3; }").arg(groove, handle, handle_hover);
+}
+
+void StyleChart(QChart *chart) {
+    chart->setTheme(dark_theme ? QChart::ChartThemeDark : QChart::ChartThemeLight);
+    if (dark_theme)
+        chart->setBackgroundBrush(QApplication::palette().color(QPalette::Base));
+}
diff --git a/viewer/ViewerTheme.h b/viewer/ViewerTheme.h
new file mode 100644
index 000000000..022c218d4
--- /dev/null
+++ b/viewer/ViewerTheme.h
@@ -0,0 +1,45 @@
+// SPDX-FileCopyrightText: 2026 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
+// SPDX-License-Identifier: GPL-3.0-only
+
+#pragma once
+
+#include <functional>
+#include <QColor>
+#include <QObject>
+#include <QString>
+
+class QChart;
+class QWidget;
+
+// The viewer's colour scheme: the light salmon one, or a dark indigo one. The View menu offers
+// System, Light and Dark; System follows the desktop where Qt can read it (Qt 6.5+) and is light
+// otherwise. A change applies at once: widgets that follow the palette repaint by themselves, and
+// the ones that put colours into stylesheets, icons or charts listen to ThemeNotifier.
+enum class ThemeChoice { System, Light, Dark };
+ThemeChoice LoadThemeChoice();
+void SetThemeChoice(ThemeChoice choice);   // stores the choice and applies it
+
+// Sets the palette and starts following the desktop's scheme. Call once, after the Fusion style is
+// set and before any window is made.
+void InitTheme();
+bool DarkTheme();
+
+class ThemeNotifier : public QObject {
+    Q_OBJECT
+public:
+    static ThemeNotifier *Instance();
+signals:
+    void changed();
+};
+
+// Sets the widget's stylesheet from `style`, now and again after every theme change.
+void SetThemedStyleSheet(QWidget *widget, const std::function<QString()> &style);
+
+// Colours that widgets put into their own stylesheets and drawings.
+QColor AccentTextColor();   // headings and headline values: navy on light, light blue on dark
+QColor IconInkColor();      // toolbar glyphs and flat toolbar-button text
+QString SliderStyle();      // the coral/navy sliders of the toolbars
+
+// Qt Charts do not follow the widget palette; this gives a chart the current scheme's colours. It
+// recolours the series too, so call it before adding series that carry colours of their own.
+void StyleChart(QChart *chart);
diff --git a/viewer/charts/JFJochDatasetInfoChartView.cpp b/viewer/charts/JFJochDatasetInfoChartView.cpp
index c22b83c44..9093c04c4 100644
--- a/viewer/charts/JFJochDatasetInfoChartView.cpp
+++ b/viewer/charts/JFJochDatasetInfoChartView.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include <QMenu>
+#include "../ViewerTheme.h"
 #include <QApplication>
 #include <QClipboard>
 #include <QCategoryAxis>
@@ -26,6 +27,12 @@ namespace {
 
 JFJochDatasetInfoChartView::JFJochDatasetInfoChartView(QWidget *parent)
     : QChartView(new QChart(), parent) {
+    StyleChart(chart());
+    // The theme recolours every series, the run colours included, so the chart is rebuilt after it.
+    connect(ThemeNotifier::Instance(), &ThemeNotifier::changed, this, [this] {
+        StyleChart(chart());
+        updateChart();
+    });
     chart()->legend()->hide();
     // Reclaim Qt Charts' outer layout padding so the axis labels keep their room even when the dock
     // is short (otherwise they are the first thing Qt drops); the inner margins are set per rebuild.
@@ -400,7 +407,7 @@ void JFJochDatasetInfoChartView::buildTimeDomainChart() {
             series->setColor(primary_color_);
         setSeriesPenWidth(series, fontMetrics().height());
         currentSeries = new QScatterSeries(this);
-        currentSeries->setColor(Qt::black);     // "current image" marker: fixed, not a run colour
+        currentSeries->setColor(palette().color(QPalette::WindowText));   // "current image" marker: not a run colour
         currentSeries->setMarkerSize(9.0);
 
         double dispMin = std::numeric_limits<double>::infinity();
diff --git a/viewer/charts/JFJochSimpleChartView.cpp b/viewer/charts/JFJochSimpleChartView.cpp
index 6a8ce3e7d..36be54608 100644
--- a/viewer/charts/JFJochSimpleChartView.cpp
+++ b/viewer/charts/JFJochSimpleChartView.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "JFJochSimpleChartView.h"
+#include "../ViewerTheme.h"
 
 #include <QMenu>
 #include <QClipboard>
@@ -12,6 +13,8 @@
 
 JFJochSimpleChartView::JFJochSimpleChartView(QWidget *parent)
     : QChartView(new QChart(), parent) {
+    StyleChart(chart());
+    connect(ThemeNotifier::Instance(), &ThemeNotifier::changed, this, [this] { StyleChart(chart()); });
     chart()->legend()->hide();
     chart()->layout()->setContentsMargins(0, 0, 0, 0);   // reclaim padding for the plot + labels
     chart()->setMargins(QMargins(2, 10, 8, 2));   // top room so the highest Y label is not clipped
diff --git a/viewer/jfjoch_viewer.cpp b/viewer/jfjoch_viewer.cpp
index 72133418d..4587560f9 100644
--- a/viewer/jfjoch_viewer.cpp
+++ b/viewer/jfjoch_viewer.cpp
@@ -10,6 +10,7 @@
 
 #include "JFJochViewerWindow.h"
 #include "RemoteDisplayMode.h"
+#include "ViewerTheme.h"
 #include "../writer/HDF5Objects.h"
 #include "../common/GitInfo.h"
 
@@ -65,28 +66,8 @@ int main(int argc, char *argv[]) {
     RegisterHDF5Filter();
     QApplication app(argc, argv);
 
-    // Theme via the palette (robust across widgets): salmon panels, white entry fields that fall back
-    // to salmon when disabled, and a navy accent for selections and progress-bar chunks.
-    // Start from Fusion's own (light) palette, not the system one, which is dark under a dark OS theme.
     app.setStyle(new SeparatorHandleStyle("Fusion"));
-    QPalette pal = app.style()->standardPalette();
-    pal.setColor(QPalette::Window, QColor(255, 235, 230));
-    pal.setColor(QPalette::Base, Qt::white);
-    pal.setColor(QPalette::AlternateBase, QColor(255, 245, 242));
-    pal.setColor(QPalette::Button, QColor(255, 235, 230));
-    pal.setColor(QPalette::Disabled, QPalette::Base, QColor(255, 235, 230));
-    pal.setColor(QPalette::Highlight, QColor(0x1f, 0x3a, 0x5f));
-    pal.setColor(QPalette::HighlightedText, Qt::white);
-    // Black text as on Linux and Windows: on macOS the palette's text colours come from the platform
-    // theme and are grey (or white under a dark OS theme). Disabled text keeps Fusion's grey.
-    for (auto group: {QPalette::Active, QPalette::Inactive})
-        for (auto role: {QPalette::WindowText, QPalette::Text, QPalette::ButtonText, QPalette::ToolTipText})
-            pal.setColor(group, role, Qt::black);
-    app.setPalette(pal);
-    // Fusion fills QGroupBox interiors with a flat light colour; make them transparent so they show
-    // the salmon window background like the rest of the UI (entry widgets stay white via the palette).
-    // The dock resize handles are drawn by SeparatorHandleStyle above, not the stylesheet.
-    app.setStyleSheet("QGroupBox { background-color: transparent; }");
+    InitTheme();   // light or dark palette and the application stylesheet, per the View menu's choice
 
     QIcon appIcon(":/jfjoch.png");
     app.setWindowIcon(appIcon);
diff --git a/viewer/toolbar/JFJochViewerToolbarDisplay.cpp b/viewer/toolbar/JFJochViewerToolbarDisplay.cpp
index d9fac3972..a0f3e91c8 100644
--- a/viewer/toolbar/JFJochViewerToolbarDisplay.cpp
+++ b/viewer/toolbar/JFJochViewerToolbarDisplay.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "JFJochViewerToolbarDisplay.h"
+#include "../ViewerTheme.h"
 #include <QLabel>
 #include "../../common/ColorScale.h"
 #include "../RemoteDisplayMode.h"
@@ -23,11 +24,6 @@ JFJochViewerToolbarDisplay::JFJochViewerToolbarDisplay(QWidget *parent)
     connect(background_tail_timer_, &QTimer::timeout,
             this, &JFJochViewerToolbarDisplay::ApplyPendingBackground);
 
-    const QString sliderStyle =
-        "QSlider::groove:horizontal { height:6px; background:#F3D9D4; border-radius:3px; }"
-        "QSlider::sub-page:horizontal { background:#FA7268; border-radius:3px; }"
-        "QSlider::handle:horizontal { background:#1F3A5F; width:14px; margin:-5px 0; border-radius:7px; }"
-        "QSlider::handle:horizontal:hover { background:#16314F; }";
     // Two sliders share the strip, so each is capped rather than expanding across the window.
     const int slider_cap = fontMetrics().averageCharWidth() * 36;
 
@@ -38,7 +34,7 @@ JFJochViewerToolbarDisplay::JFJochViewerToolbarDisplay(QWidget *parent)
     foreground_slider->setValue(10);
     foreground_slider->setToolTip("White point of the color map (image contrast); hold F and use"
                                   " the mouse wheel, Ctrl+wheel for coarse steps");
-    foreground_slider->setStyleSheet(sliderStyle);
+    SetThemedStyleSheet(foreground_slider, SliderStyle);
     foreground_slider->setMaximumWidth(slider_cap);
     addWidget(foreground_slider);
 
@@ -49,7 +45,7 @@ JFJochViewerToolbarDisplay::JFJochViewerToolbarDisplay(QWidget *parent)
     background_slider->setValue(0);
     background_slider->setToolTip("Black point of the color map (0 = off); hold B and use"
                                   " the mouse wheel");
-    background_slider->setStyleSheet(sliderStyle);
+    SetThemedStyleSheet(background_slider, SliderStyle);
     background_slider->setMaximumWidth(slider_cap);
     addWidget(background_slider);
 
@@ -60,7 +56,7 @@ JFJochViewerToolbarDisplay::JFJochViewerToolbarDisplay(QWidget *parent)
         b->setCheckable(true);
         b->setToolTip(tip);
         b->setCursor(Qt::PointingHandCursor);
-        b->setStyleSheet(ToolbarIcons::buttonStyle());
+        SetThemedStyleSheet(b, ToolbarIcons::buttonStyle);
         return b;
     };
     auto_foreground_button = makeToggle("Auto", "Auto-contrast to the current image");
diff --git a/viewer/toolbar/JFJochViewerToolbarImage.cpp b/viewer/toolbar/JFJochViewerToolbarImage.cpp
index 3362a8905..c9b12068f 100644
--- a/viewer/toolbar/JFJochViewerToolbarImage.cpp
+++ b/viewer/toolbar/JFJochViewerToolbarImage.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "JFJochViewerToolbarImage.h"
+#include "../ViewerTheme.h"
 #include <algorithm>
 #include <QTimer>
 #include "../RemoteDisplayMode.h"
@@ -18,7 +19,7 @@ JFJochViewerToolbarImage::JFJochViewerToolbarImage(QWidget *parent) : QToolBar(p
         b->setToolTip(tip);
         b->setCheckable(checkable);
         b->setCursor(Qt::PointingHandCursor);
-        b->setStyleSheet(ToolbarIcons::buttonStyle());
+        SetThemedStyleSheet(b, ToolbarIcons::buttonStyle);
         return b;
     };
 
@@ -38,11 +39,7 @@ JFJochViewerToolbarImage::JFJochViewerToolbarImage(QWidget *parent) : QToolBar(p
     image_number_slider->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Fixed);
     image_number_slider->setMinimumWidth(140);
     image_number_slider->setToolTip("Scrub through the dataset");
-    image_number_slider->setStyleSheet(
-        "QSlider::groove:horizontal { height:6px; background:#F3D9D4; border-radius:3px; }"
-        "QSlider::sub-page:horizontal { background:#FA7268; border-radius:3px; }"
-        "QSlider::handle:horizontal { background:#1F3A5F; width:14px; margin:-5px 0; border-radius:7px; }"
-        "QSlider::handle:horizontal:hover { background:#16314F; }");
+    SetThemedStyleSheet(image_number_slider, SliderStyle);
     image_number_slider_timer = new QTimer(this);
     image_number_slider_timer->setSingleShot(true);
 
diff --git a/viewer/widgets/CollapsibleSection.cpp b/viewer/widgets/CollapsibleSection.cpp
index f60602089..e7eb2a130 100644
--- a/viewer/widgets/CollapsibleSection.cpp
+++ b/viewer/widgets/CollapsibleSection.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "CollapsibleSection.h"
+#include "../ViewerTheme.h"
 
 #include <QToolButton>
 #include <QVBoxLayout>
@@ -18,10 +19,12 @@ CollapsibleSection::CollapsibleSection(const QString &title, QWidget *parent) :
     header_->setArrowType(Qt::RightArrow);
     header_->setCursor(Qt::PointingHandCursor);
     header_->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Fixed);
-    header_->setStyleSheet(
-        "QToolButton { color:#1F3A5F; font-weight:bold; text-align:left;"
-        " padding:3px 6px 3px 6px; border:none;"
-        " border-left:3px solid #FA7268; border-bottom:1px solid #FA7268; background:transparent; }");
+    SetThemedStyleSheet(header_, [] {
+        return QString("QToolButton { color:%1; font-weight:bold; text-align:left;"
+                       " padding:3px 6px 3px 6px; border:none;"
+                       " border-left:3px solid #FA7268; border-bottom:1px solid #FA7268; background:transparent; }")
+            .arg(AccentTextColor().name());
+    });
 
     content_ = new QWidget(this);
     content_->setVisible(false);
diff --git a/viewer/widgets/JFJochViewerImageStatistics.cpp b/viewer/widgets/JFJochViewerImageStatistics.cpp
index 51ae89045..a1b816586 100644
--- a/viewer/widgets/JFJochViewerImageStatistics.cpp
+++ b/viewer/widgets/JFJochViewerImageStatistics.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "JFJochViewerImageStatistics.h"
+#include "../ViewerTheme.h"
 #include "../../common/time_utc.h"
 #include <QFormLayout>
 #include <QRegularExpression>
@@ -403,7 +404,7 @@ void JFJochViewerImageStatistics::loadImage(std::shared_ptr<const JFJochReaderIm
 
         auto latt = image->ImageData().indexing_lattice;
         if (latt) {
-            text = mkUnitCell(latt->GetUnitCell(), "purple");
+            text = mkUnitCell(latt->GetUnitCell(), DarkTheme() ? "violet" : "purple");
 
             auto vec0 = latt->Vec0();
             auto vec1 = latt->Vec1();
diff --git a/viewer/widgets/JFJochViewerROIList.cpp b/viewer/widgets/JFJochViewerROIList.cpp
index 2fbdc395e..78800a731 100644
--- a/viewer/widgets/JFJochViewerROIList.cpp
+++ b/viewer/widgets/JFJochViewerROIList.cpp
@@ -54,7 +54,6 @@ JFJochViewerROIList::JFJochViewerROIList(QWidget *parent) : QWidget(parent) {
         fedit_[i] = new QLineEdit(this);
         fedit_[i]->setAlignment(Qt::AlignRight);
         fedit_[i]->setMaximumWidth(90);
-        fedit_[i]->setStyleSheet("background-color: white; color: black;");
         grid->addWidget(flabel_[i], i, 0);
         grid->addWidget(fedit_[i], i, 1, Qt::AlignRight);
         connect(fedit_[i], &QLineEdit::editingFinished, this, &JFJochViewerROIList::ApplyEditor);
diff --git a/viewer/widgets/NumberLineEdit.cpp b/viewer/widgets/NumberLineEdit.cpp
index de6939e5a..8727ac0e2 100644
--- a/viewer/widgets/NumberLineEdit.cpp
+++ b/viewer/widgets/NumberLineEdit.cpp
@@ -13,7 +13,6 @@ NumberLineEdit::NumberLineEdit(float minVal,
         , decimalCount(decimals)
         , savedValue(def_value) {
 
-    setStyleSheet("background-color: rgb(255, 255, 255);");
     setRange(minVal, maxVal);
     setDecimals(decimalCount);
     setButtonSymbols(QAbstractSpinBox::ButtonSymbols::NoButtons);
diff --git a/viewer/widgets/NumericComboBox.cpp b/viewer/widgets/NumericComboBox.cpp
index 688dfb155..caf7159bd 100644
--- a/viewer/widgets/NumericComboBox.cpp
+++ b/viewer/widgets/NumericComboBox.cpp
@@ -16,8 +16,6 @@ NumericComboBox::NumericComboBox(std::vector<int> options, int min, int max, QWi
 
     setCurrentText(QString::number(min));
 
-    setStyleSheet("background-color: rgb(255, 255, 255);");
-
     connect(this, QOverload<int>::of(&QComboBox::currentIndexChanged),
             this, &NumericComboBox::onCurrentIndexChanged);
     connect(this, &QComboBox::editTextChanged,
@@ -35,7 +33,7 @@ void NumericComboBox::setValue(int value) {
 
 void NumericComboBox::onCurrentIndexChanged(int index) {
     if (index >= 0) {
-        setStyleSheet("background-color: rgb(255, 255, 255);");
+        setStyleSheet("");
         emit valueChanged(getValue());
     }
 }
@@ -44,7 +42,7 @@ void NumericComboBox::onEditTextChanged(const QString &text) {
     bool ok;
     int value = text.toInt(&ok);
     if (ok) {
-        setStyleSheet("background-color: rgb(255, 255, 255);");
+        setStyleSheet("");
         emit valueChanged(value);
     } else {
         setStyleSheet("background-color: rgb(125, 0, 0);");
diff --git a/viewer/widgets/SliderPlusBox.cpp b/viewer/widgets/SliderPlusBox.cpp
index 98e06cd09..b81776346 100644
--- a/viewer/widgets/SliderPlusBox.cpp
+++ b/viewer/widgets/SliderPlusBox.cpp
@@ -33,7 +33,6 @@ SliderPlusBox::SliderPlusBox(double min, double max, double step, int decimals,
     m_doubleSpinBox->setDecimals(decimals);
     m_doubleSpinBox->setSingleStep(step);
     m_doubleSpinBox->setValue(v);
-    m_doubleSpinBox->setStyleSheet("background-color: rgb(255, 255, 255);");
 
     m_updateTimer = new QTimer(this);
     m_updateTimer->setInterval(500);  // 500ms throttle
diff --git a/viewer/widgets/TitleLabel.cpp b/viewer/widgets/TitleLabel.cpp
index 2f4ded107..fa9ffc986 100644
--- a/viewer/widgets/TitleLabel.cpp
+++ b/viewer/widgets/TitleLabel.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "TitleLabel.h"
+#include "../ViewerTheme.h"
 
 TitleLabel::TitleLabel(QString text, QWidget *parent) : QLabel(parent) {
     // Slim section header: navy bold text with a coral accent rule, in place of a heavy 50px
@@ -10,12 +11,14 @@ TitleLabel::TitleLabel(QString text, QWidget *parent) : QLabel(parent) {
     QFont f = font();
     f.setBold(true);
     setFont(f);
-    setStyleSheet("QLabel {"
-                  " color: #1F3A5F;"
-                  " background-color: transparent;"
-                  " border-left: 3px solid #FA7268;"
-                  " border-bottom: 1px solid #FA7268;"
-                  " padding: 3px 6px 3px 8px; }");
+    SetThemedStyleSheet(this, [] {
+        return QString("QLabel {"
+                       " color: %1;"
+                       " background-color: transparent;"
+                       " border-left: 3px solid #FA7268;"
+                       " border-bottom: 1px solid #FA7268;"
+                       " padding: 3px 6px 3px 8px; }").arg(AccentTextColor().name());
+    });
     setAlignment(Qt::AlignLeft | Qt::AlignVCenter);
     setFixedHeight(26);
 }
diff --git a/viewer/widgets/ToolbarIcons.cpp b/viewer/widgets/ToolbarIcons.cpp
index 3a5241ee7..b5d8b38db 100644
--- a/viewer/widgets/ToolbarIcons.cpp
+++ b/viewer/widgets/ToolbarIcons.cpp
@@ -2,7 +2,9 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "ToolbarIcons.h"
+#include "../ViewerTheme.h"
 
+#include <QIconEngine>
 #include <QPainter>
 #include <QPainterPath>
 #include <QPolygonF>
@@ -10,28 +12,49 @@
 
 namespace {
     constexpr int S = 32;
-    const QColor INK("#2B2B2B");
     const QColor DISABLED("#9AA6B3");
 
     using DrawFn = std::function<void(QPainter &, const QColor &)>;
 
-    QPixmap render(const DrawFn &fn, const QColor &color) {
-        QPixmap pm(S, S);
+    // The glyphs are drawn on an S x S canvas; the painter is scaled so they come out sharp at any size.
+    QPixmap render(const DrawFn &fn, const QColor &color, const QSize &size) {
+        QPixmap pm(size);
         pm.fill(Qt::transparent);
         QPainter p(&pm);
         p.setRenderHint(QPainter::Antialiasing);
+        p.scale(size.width() / double(S), size.height() / double(S));
         fn(p, color);
         return pm;
     }
 
-    // Toggle-capable icon: ink when off, white when on (checked, navy background), grey when disabled.
+    // Draws the glyph when the icon is painted rather than when it is made, so the ink follows the
+    // theme: ink when off, white when on (a toggle on its navy checked background), grey when disabled.
+    class InkIconEngine final : public QIconEngine {
+    public:
+        InkIconEngine(DrawFn fn, bool toggle) : fn_(std::move(fn)), toggle_(toggle) {}
+
+        void paint(QPainter *painter, const QRect &rect, QIcon::Mode mode, QIcon::State state) override {
+            painter->drawPixmap(rect, pixmap(rect.size(), mode, state));
+        }
+
+        QPixmap pixmap(const QSize &size, QIcon::Mode mode, QIcon::State state) override {
+            QColor color = IconInkColor();
+            if (mode == QIcon::Disabled)
+                color = DISABLED;
+            else if (toggle_ && state == QIcon::On)
+                color = Qt::white;
+            return render(fn_, color, size);
+        }
+
+        QIconEngine *clone() const override { return new InkIconEngine(fn_, toggle_); }
+
+    private:
+        DrawFn fn_;
+        bool toggle_;
+    };
+
     QIcon toggleIcon(const DrawFn &fn) {
-        QIcon ic;
-        ic.addPixmap(render(fn, INK), QIcon::Normal, QIcon::Off);
-        ic.addPixmap(render(fn, Qt::white), QIcon::Normal, QIcon::On);
-        ic.addPixmap(render(fn, Qt::white), QIcon::Active, QIcon::On);
-        ic.addPixmap(render(fn, DISABLED), QIcon::Disabled, QIcon::Off);
-        return ic;
+        return QIcon(new InkIconEngine(fn, true));
     }
 
     void fillTriangle(QPainter &p, const QColor &c, QPointF a, QPointF b, QPointF tip) {
@@ -180,19 +203,16 @@ QIcon httpSync(HttpState state) {
         p.setBrush(dot);
         p.drawEllipse(QPointF(24, 24), 4.2, 4.2);   // status dot
     };
-    QIcon ic;
-    ic.addPixmap(render(draw, INK), QIcon::Normal, QIcon::Off);
-    ic.addPixmap(render(draw, DISABLED), QIcon::Disabled, QIcon::Off);
-    return ic;
+    return QIcon(new InkIconEngine(draw, false));
 }
 
 QString buttonStyle() {
-    return QStringLiteral(
-        "QToolButton { border:none; border-radius:4px; padding:4px 6px; color:#2B2B2B;"
+    return QString(
+        "QToolButton { border:none; border-radius:4px; padding:4px 6px; color:%1;"
         " background:transparent; }"
         " QToolButton:hover { background:rgba(250,114,104,0.20); }"
         " QToolButton:checked { background:#1F3A5F; color:white; }"
-        " QToolButton:disabled { color:#9AA6B3; }");
+        " QToolButton:disabled { color:#9AA6B3; }").arg(IconInkColor().name());
 }
 
 }  // namespace ToolbarIcons
diff --git a/viewer/windows/JFJochCalibrationResultWindow.cpp b/viewer/windows/JFJochCalibrationResultWindow.cpp
index 33f042077..afaf1e56d 100644
--- a/viewer/windows/JFJochCalibrationResultWindow.cpp
+++ b/viewer/windows/JFJochCalibrationResultWindow.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "JFJochCalibrationResultWindow.h"
+#include "../ViewerTheme.h"
 
 #include <QFrame>
 #include <QHBoxLayout>
@@ -27,7 +28,7 @@ namespace {
         f.setPointSizeF(f.pointSizeF() * 2.2);
         f.setBold(true);
         v->setFont(f);
-        v->setStyleSheet("color: #1F3A5F;");
+        SetThemedStyleSheet(v, [] { return "color: " + AccentTextColor().name() + ";"; });
         v->setAlignment(Qt::AlignCenter);
         auto *c = new QLabel(caption, card);
         c->setStyleSheet("color: gray;");
diff --git a/viewer/windows/JFJochMergeStatsWindow.cpp b/viewer/windows/JFJochMergeStatsWindow.cpp
index d04de09a0..db7c50401 100644
--- a/viewer/windows/JFJochMergeStatsWindow.cpp
+++ b/viewer/windows/JFJochMergeStatsWindow.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-only
 
 #include "JFJochMergeStatsWindow.h"
+#include "../ViewerTheme.h"
 
 #include <cmath>
 #include <limits>
@@ -57,7 +58,7 @@ namespace {
         f.setPointSizeF(f.pointSizeF() * 2.2);
         f.setBold(true);
         v->setFont(f);
-        v->setStyleSheet("color: #1F3A5F;");
+        SetThemedStyleSheet(v, [] { return "color: " + AccentTextColor().name() + ";"; });
         v->setAlignment(Qt::AlignCenter);
         auto *c = new QLabel(caption, card);
         c->setStyleSheet("color: gray;");
@@ -170,7 +171,10 @@ JFJochMergeStatsWindow::JFJochMergeStatsWindow(const QString &title, const Merge
         auto *banner = new QLabel(text, this);
         banner->setContentsMargins(10, 6, 10, 6);
         if (twinning.twinning_suspected)
-            banner->setStyleSheet("color: rgb(160,0,0); border: 1px solid red; border-radius: 4px;");
+            SetThemedStyleSheet(banner, [] {
+                return QString("color: %1; border: 1px solid red; border-radius: 4px;")
+                    .arg(DarkTheme() ? "#FF7B72" : "rgb(160,0,0)");
+            });
         else
             banner->setStyleSheet("color: gray;");
         layout->addWidget(banner);
@@ -183,22 +187,22 @@ JFJochMergeStatsWindow::JFJochMergeStatsWindow(const QString &title, const Merge
     plotBtn->setCheckable(true);
     plotBtn->setChecked(true);
     plotBtn->setToolTip("Per-resolution plot");
-    plotBtn->setStyleSheet(ToolbarIcons::buttonStyle());
+    SetThemedStyleSheet(plotBtn, ToolbarIcons::buttonStyle);
     auto *tableBtn = new QToolButton(this);
     tableBtn->setIcon(ToolbarIcons::table());
     tableBtn->setCheckable(true);
     tableBtn->setToolTip("Per-shell table");
-    tableBtn->setStyleSheet(ToolbarIcons::buttonStyle());
+    SetThemedStyleSheet(tableBtn, ToolbarIcons::buttonStyle);
     auto *isaBtn = new QToolButton(this);
     isaBtn->setIcon(ToolbarIcons::linePlot());
     isaBtn->setCheckable(true);
     isaBtn->setToolTip("ISa diagnostic: merged I/σ against I");
-    isaBtn->setStyleSheet(ToolbarIcons::buttonStyle());
+    SetThemedStyleSheet(isaBtn, ToolbarIcons::buttonStyle);
     auto *reportBtn = new QToolButton(this);
     reportBtn->setText("Report");
     reportBtn->setCheckable(true);
     reportBtn->setToolTip("The rugnux result report, as written to <prefix>_report.txt");
-    reportBtn->setStyleSheet(ToolbarIcons::buttonStyle());
+    SetThemedStyleSheet(reportBtn, ToolbarIcons::buttonStyle);
     auto *viewGroup = new QButtonGroup(this);
     viewGroup->setExclusive(true);
     viewGroup->addButton(plotBtn, 0);
@@ -277,7 +281,9 @@ QWidget *JFJochMergeStatsWindow::buildIsaDiagnostic(const std::vector<std::pair<
     points->setName("merged reflections");
     points->setMarkerSize(4.0);
     points->setBorderColor(Qt::transparent);
-    points->setColor(QColor(0x1F, 0x3A, 0x5F, 0x60));
+    QColor point_color = AccentTextColor();
+    point_color.setAlpha(0x60);
+    points->setColor(point_color);
 
     double i_lo = std::numeric_limits<double>::max(), i_hi = 0.0, r_hi = 0.0;
     for (const auto &[I, sigma] : i_sigma) {
@@ -295,6 +301,7 @@ QWidget *JFJochMergeStatsWindow::buildIsaDiagnostic(const std::vector<std::pair<
     }
 
     auto *chart = new QChart();
+    StyleChart(chart);
     chart->legend()->setVisible(false);
     chart->addSeries(points);
 
diff --git a/viewer/windows/JFJochViewerImageListWindow.cpp b/viewer/windows/JFJochViewerImageListWindow.cpp
index 10b13c4fd..909117cde 100644
--- a/viewer/windows/JFJochViewerImageListWindow.cpp
+++ b/viewer/windows/JFJochViewerImageListWindow.cpp
@@ -223,7 +223,7 @@ void JFJochViewerImageListWindow::imageLoaded(std::shared_ptr<const JFJochReader
 
     // Only the row losing the highlight and the row gaining it change. Repainting every cell
     // emitted dataChanged for the whole model - and a proxy re-sort - on each frame.
-    setRowBackground(highlighted_row, Qt::white);
+    setRowBackground(highlighted_row, QBrush());   // back to the palette's own
     setRowBackground(curr_row, Qt::blue);
     highlighted_row = curr_row;
 }
diff --git a/viewer/windows/JFJochViewerReflectionListWindow.cpp b/viewer/windows/JFJochViewerReflectionListWindow.cpp
index e564e6e34..3ff41d5d5 100644
--- a/viewer/windows/JFJochViewerReflectionListWindow.cpp
+++ b/viewer/windows/JFJochViewerReflectionListWindow.cpp
@@ -29,7 +29,6 @@ JFJochViewerReflectionListWindow::JFJochViewerReflectionListWindow(QWidget* pare
     tableView->verticalHeader()->setVisible(false);
     tableView->horizontalHeader()->setSectionsClickable(true);
     tableView->horizontalHeader()->setSortIndicatorShown(true);
-    tableView->setStyleSheet("background-color: white;");
     tableView->setSelectionBehavior(QAbstractItemView::SelectRows);
     tableView->setSelectionMode(QAbstractItemView::SingleSelection);
 
diff --git a/viewer/windows/JFJochViewerSpotListWindow.cpp b/viewer/windows/JFJochViewerSpotListWindow.cpp
index 300a85984..2ab0aaedf 100644
--- a/viewer/windows/JFJochViewerSpotListWindow.cpp
+++ b/viewer/windows/JFJochViewerSpotListWindow.cpp
@@ -29,7 +29,6 @@ JFJochViewerSpotListWindow::JFJochViewerSpotListWindow(QWidget* parent)
     tableView->verticalHeader()->setVisible(false);
     tableView->horizontalHeader()->setSectionsClickable(true);
     tableView->horizontalHeader()->setSortIndicatorShown(true);
-    tableView->setStyleSheet("background-color: white;");
 
     tableView->setSelectionBehavior(QAbstractItemView::SelectRows);
     tableView->setSelectionMode(QAbstractItemView::SingleSelection);