diff --git a/src/calibre/gui2/__init__.py b/src/calibre/gui2/__init__.py
index 38dd4f707a..d825fb1e1f 100644
--- a/src/calibre/gui2/__init__.py
+++ b/src/calibre/gui2/__init__.py
@@ -669,8 +669,7 @@ else:
 def is_dark_theme():
     pal = QApplication.instance().palette()
     col = pal.color(pal.Window)
-    h, s, v, a = col.getHsvF()
-    return v < 0.45
+    return max(col.getRgb()[:3]) < 115
 
 
 def choose_osx_app(window, name, title, default_dir='/Applications'):
diff --git a/src/calibre/gui2/progress_indicator/QProgressIndicator.cpp b/src/calibre/gui2/progress_indicator/QProgressIndicator.cpp
index bf8b11f67b..eca591fa75 100644
--- a/src/calibre/gui2/progress_indicator/QProgressIndicator.cpp
+++ b/src/calibre/gui2/progress_indicator/QProgressIndicator.cpp
@@ -133,6 +133,13 @@ static inline QByteArray detectDesktopEnvironment()
     return QByteArrayLiteral("UNKNOWN");
 }
 
+static inline bool
+is_color_dark(const QColor &col) {
+	int r, g, b;
+	col.getRgb(&r, &g, &b);
+	return r < 115 && g < 155 && b < 115;
+}
+
 class CalibreStyle: public QProxyStyle {
     private:
         QHash<int, QString> icon_map;
@@ -213,14 +220,14 @@ class CalibreStyle: public QProxyStyle {
 					if (const QStyleOptionTabBarBase *tbb = qstyleoption_cast<const QStyleOptionTabBarBase *>(option)) {
 						if (tbb->shape == QTabBar::RoundedNorth) {
 							QColor bg = option->palette.color(QPalette::Window);
-							if (bg.valueF() < 0.45) return;
+							if (is_color_dark(bg)) return;
 						}
 					}
 					break; // }}}
 
 				case PE_IndicatorCheckBox: // {{{
 					// Fix color used to draw checkbox outline in dark mode
-					if (option->palette.color(QPalette::Window).valueF() < 0.45) {
+					if (is_color_dark(option->palette.color(QPalette::Window))) {
 						baseStyle()->drawPrimitive(element, option, painter, widget);
 						painter->save();
 						painter->translate(0.5, 0.5);
@@ -254,7 +261,7 @@ class CalibreStyle: public QProxyStyle {
 						const int margin = 6;
 						QColor bg = option->palette.color(QPalette::Window);
 						QColor first, second;
-						if (bg.valueF() < 0.45) {
+						if (is_color_dark(bg)) {
 							first = bg.darker(115);
 							second = bg.lighter(115);
 						} else {
@@ -311,7 +318,7 @@ class CalibreStyle: public QProxyStyle {
 										QPalette::Text);
 								w = menuItem->fontMetrics.horizontalAdvance(menuItem->text) + margin;
 							}
-							if (menuItem->palette.color(QPalette::Window).valueF() < 0.45) painter->setPen(Qt::gray);
+							if (is_color_dark(menuItem->palette.color(QPalette::Window))) painter->setPen(Qt::gray);
 							else painter->setPen(QColor(0, 0, 0, 60).lighter(106));
 							bool reverse = menuItem->direction == Qt::RightToLeft;
 							painter->drawLine(menuItem->rect.left() + margin + (reverse ? 0 : w), menuItem->rect.center().y(),
diff --git a/src/pyj/read_book/view.pyj b/src/pyj/read_book/view.pyj
index 60d39b9317..a4312f6e32 100644
--- a/src/pyj/read_book/view.pyj
+++ b/src/pyj/read_book/view.pyj
@@ -681,8 +681,9 @@ class View:
         bg_image_fade = 'transparent'
         cs = self.get_color_scheme(True)
         fade = int(sd.get('background_image_fade'))
+        rgba = cached_color_to_rgba(cs.background)
+        is_dark_theme = max(rgba[0], rgba[1], rgba[2]) < 115
         if self.iframe.style.backgroundImage is not 'none' and fade > 0:
-            rgba = cached_color_to_rgba(cs.background)
             bg_image_fade = f'rgba({rgba[0]}, {rgba[1]}, {rgba[2]}, {fade/100})'
         return {
             'margin_left': 0 if name is self.book.manifest.title_page_name else sd.get('margin_left'),
@@ -690,6 +691,7 @@ class View:
             'read_mode': sd.get('read_mode'),
             'columns_per_screen': sd.get('columns_per_screen'),
             'color_scheme': cs,
+            'is_dark_theme': is_dark_theme,
             'bg_image_fade': bg_image_fade,
             'base_font_size': sd.get('base_font_size'),
             'user_stylesheet': sd.get('user_stylesheet'),