Theme: Use dark theme by default on dark windows

Change-Id: Ieb024e01adfdce1c37948097bae85fa2488ee341
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/theme/theme.cpp

@@ -26,6 +26,7 @@
 #include "theme.h"
 #include "theme_p.h"
 #include "../algorithm.h"
+#include "../hostosinfo.h"
 #include "../qtcassert.h"
 #ifdef Q_OS_MACOS
 #import "theme_mac.h"
@@ -242,6 +243,18 @@ void Theme::readSettings(QSettings &settings)
     }
 }
 
+bool Theme::systemUsesDarkMode()
+{
+    if (HostOsInfo::isWindowsHost()) {
+        constexpr char regkey[]
+            = "HKEY_CURRENT_USER\\Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize";
+        bool ok;
+        const auto setting = QSettings(regkey, QSettings::NativeFormat).value("AppsUseLightTheme").toInt(&ok);
+        return ok && setting == 0;
+    }
+    return false;
+}
+
 // If you copy QPalette, default values stay at default, even if that default is different
 // within the context of different widgets. Create deep copy.
 static QPalette copyPalette(const QPalette &p)

src/libs/utils/theme/theme.h

@@ -394,6 +394,7 @@ public:
 
     void readSettings(QSettings &settings);
 
+    static bool systemUsesDarkMode();
     static QPalette initialPalette();
 
 protected:

src/plugins/coreplugin/coreconstants.h

@@ -220,6 +220,7 @@ const char SETTINGS_DEFAULT_LINE_TERMINATOR[] = "General/DefaultLineTerminator";
 
 const char SETTINGS_THEME[] = "Core/CreatorTheme";
 const char DEFAULT_THEME[] = "flat";
+const char DEFAULT_DARK_THEME[] = "flat-dark";
 
 const char TR_CLEAR_MENU[]         = QT_TRANSLATE_NOOP("Core", "Clear Menu");
 

src/plugins/coreplugin/themechooser.cpp

@@ -225,9 +225,10 @@ QList<ThemeEntry> ThemeEntry::availableThemes()
 
 Id ThemeEntry::themeSetting()
 {
-    const Id setting =
-            Id::fromSetting(ICore::settings()->value(QLatin1String(Constants::SETTINGS_THEME),
-                                                     QLatin1String(Constants::DEFAULT_THEME)));
+    auto defaultId = Theme::systemUsesDarkMode() ? Constants::DEFAULT_DARK_THEME
+                                                 : Constants::DEFAULT_THEME;
+    const Id setting = Id::fromSetting(
+        ICore::settings()->value(Constants::SETTINGS_THEME, defaultId));
 
     const QList<ThemeEntry> themes = availableThemes();
     if (themes.empty())