Bug 1700294 - Introduce LookAndFeel::SystemColorScheme(). r=mstange

Trivial, but maybe worth splitting.

Differential Revision: https://phabricator.services.mozilla.com/D110727

widget/LookAndFeel.h

@@ -425,6 +425,11 @@ class LookAndFeel {
   // This is currently ignored (but won't be for long).
   enum class ColorScheme : uint8_t { Light, Dark };
 
+  static ColorScheme SystemColorScheme() {
+    return GetInt(IntID::SystemUsesDarkTheme) ? ColorScheme::Dark
+                                              : ColorScheme::Light;
+  }
+
   // Whether standins for native colors should be used (that is, colors faked,
   // taken from win7, mostly). This forces light appearance, effectively.
   enum class UseStandins : bool { No, Yes };