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/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);");