Gui: Add ThemeTokenManager class to contain theme parameters

This class aims to implement Design Token idea into FreeCAD themes. It
allows themes to use generic variables with generic values so we could
use one qss theme and change the style based on values from preference
packs.

src/Gui/OverlayManager.cpp

@@ -133,6 +133,8 @@ public:
         if (activeStyleSheet.isEmpty()) {
             activeStyleSheet = _default;
         }
+
+        activeStyleSheet = Application::Instance->replaceVariablesInQss(activeStyleSheet);
     }
 
     ParameterGrp::handle handle;
@@ -157,7 +159,7 @@ private:
         }
         else if (!overlayStyleSheet.isEmpty() && !QFile::exists(overlayStyleSheet)) {
             // User did choose one of predefined stylesheets, we need to qualify it with namespace
-                overlayStyleSheet = QStringLiteral("overlay:%1").arg(overlayStyleSheet);
+            overlayStyleSheet = QStringLiteral("overlay:%1").arg(overlayStyleSheet);
         }
 
         return overlayStyleSheet;