fix: do not apply system theme on launch

Closes PrismLauncher/PrismLauncher#490

Regression introduced by PrismLauncher/PrismLauncher#249

Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>

launcher/ui/themes/SystemTheme.cpp

@@ -60,9 +60,13 @@ SystemTheme::SystemTheme()
     themeDebugLog() << "System theme not found, defaulted to Fusion";
 }
 
-void SystemTheme::apply()
+void SystemTheme::apply(bool initial)
 {
-    ITheme::apply();
+    // See https://github.com/MultiMC/Launcher/issues/1790
+    // or https://github.com/PrismLauncher/PrismLauncher/issues/490
+    if (initial)
+        return; 
+    ITheme::apply(initial);
 }
 
 QString SystemTheme::id()