Add QPlatformTheme::Appearance for detecting light/dark modes

And implement it on Windows and macOS.

Fixes: QTBUG-83908
Fixes: QTBUG-94859
Change-Id: I7b0c062adf5d4dbaefa64c862ab8ab1348809d71
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qplatformtheme.cpp

@@ -401,6 +401,11 @@ QPlatformDialogHelper *QPlatformTheme::createPlatformDialogHelper(DialogType typ
     return nullptr;
 }
 
+QPlatformTheme::Appearance QPlatformTheme::appearance() const
+{
+    return Appearance::Unknown;
+}
+
 const QPalette *QPlatformTheme::palette(Palette type) const
 {
     Q_D(const QPlatformTheme);

src/gui/kernel/qplatformtheme.h

@@ -128,6 +128,12 @@ public:
         MessageDialog
     };
 
+    enum class Appearance {
+        Unknown = 0x0000,
+        Light = 0x0001,
+        Dark = 0x0002
+    };
+
     enum Palette {
         SystemPalette,
         ToolTipPalette,
@@ -299,6 +305,8 @@ public:
     virtual QPlatformSystemTrayIcon *createPlatformSystemTrayIcon() const;
 #endif
 
+    virtual Appearance appearance() const;
+
     virtual const QPalette *palette(Palette type = SystemPalette) const;
 
     virtual const QFont *font(Font type = SystemFont) const;

src/plugins/platforms/cocoa/qcocoatheme.h

@@ -73,6 +73,7 @@ public:
     QIcon fileIcon(const QFileInfo &fileInfo, QPlatformTheme::IconOptions options = {}) const override;
 
     QVariant themeHint(ThemeHint hint) const override;
+    Appearance appearance() const override;
     QString standardButtonText(int button) const override;
     QKeySequence standardButtonShortcut(int button) const override;
 

src/plugins/platforms/cocoa/qcocoatheme.mm

@@ -533,6 +533,11 @@ QVariant QCocoaTheme::themeHint(ThemeHint hint) const
     return QPlatformTheme::themeHint(hint);
 }
 
+QPlatformTheme::Appearance QCocoaTheme::appearance() const
+{
+    return qt_mac_applicationIsInDarkMode() ? Appearance::Dark : Appearance::Light;
+}
+
 QString QCocoaTheme::standardButtonText(int button) const
 {
     return button == QPlatformDialogHelper::Discard ?

src/plugins/platforms/windows/qwindowstheme.cpp

@@ -529,6 +529,11 @@ QVariant QWindowsTheme::themeHint(ThemeHint hint) const
     return QPlatformTheme::themeHint(hint);
 }
 
+QPlatformTheme::Appearance QWindowsTheme::appearance() const
+{
+    return QWindowsContext::isDarkMode() ? Appearance::Dark : Appearance::Light;
+}
+
 void QWindowsTheme::clearPalettes()
 {
     qDeleteAll(m_palettes, m_palettes + NPalettes);

src/plugins/platforms/windows/qwindowstheme.h

@@ -64,6 +64,9 @@ public:
     QPlatformSystemTrayIcon *createPlatformSystemTrayIcon() const override;
 #endif
     QVariant themeHint(ThemeHint) const override;
+
+    Appearance appearance() const override;
+
     const QPalette *palette(Palette type = SystemPalette) const override
         { return m_palettes[type]; }
     const QFont *font(Font type = SystemFont) const override