Use registry-based dark mode detection on <= Windows 10

Signed-off-by: Claudio Cambra <claudio.cambra@nextcloud.com>

src/libsync/theme.cpp

@@ -1022,6 +1022,7 @@ QVariantMap Theme::systemPalette()
 bool Theme::darkMode()
 {
     connectToPaletteSignal();
+    const auto isDarkFromStyle = [] {
         switch (qGuiApp->styleHints()->colorScheme())
         {
         case Qt::ColorScheme::Dark:
@@ -1033,6 +1034,17 @@ bool Theme::darkMode()
         }
 
         return false;
+    };
+
+#ifdef Q_OS_WIN
+    static const auto darkModeSubkey = QStringLiteral("Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize");
+    if (!IsWindows11OrGreater() &&
+        Utility::registryKeyExists(HKEY_CURRENT_USER, darkModeSubkey) &&
+        !Utility::registryGetKeyValue(HKEY_CURRENT_USER, darkModeSubkey, QStringLiteral("AppsUseLightTheme")).toBool()) {
+        return true;
+    }
+#endif
+    return isDarkFromStyle();
 }
 
 void Theme::setOverrideServerUrl(const QString &overrideServerUrl)