Viewer: dark theme, switchable live from the View menu

View > Theme offers Follow system / Light / Dark, stored in the settings.
Follow system uses the desktop's colour scheme where Qt reports it (6.5+)
and is light otherwise. The dark theme is a deep indigo (#1A1D3A panels,
#12142B entry fields and charts).

The palettes live in the new ViewerTheme module. A switch applies at once:
the application stylesheet is set again after the palette so every widget
is re-polished (with a stylesheet active, polished widgets otherwise keep
their old palette); widgets that bake colours into stylesheets use
SetThemedStyleSheet, which re-applies them on ThemeNotifier::changed;
toolbar icons draw their glyph at paint time through an icon engine, so the
ink follows the theme (and stays sharp at any size); charts re-theme and
rebuild. Hard-coded white backgrounds on entry fields and tables are
removed in favour of the palette's base colour.

The idle detector-status badge in the status bar is now transparent instead
of an empty default-styled progress bar.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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);
 }