[fix:] fixed several issues and improved performance as well as ux

App.axaml.cs

@@ -1,6 +1,8 @@
 using Avalonia;
+using Avalonia.Controls;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Markup.Xaml;
+using Avalonia.Styling;
 
 namespace PLG_Exam;
 
@@ -11,6 +13,13 @@ public partial class App : Application
         AvaloniaXamlLoader.Load(this);
     }
 
+    public void ToggleTheme()
+    {
+        RequestedThemeVariant = RequestedThemeVariant == ThemeVariant.Dark 
+            ? ThemeVariant.Light 
+            : ThemeVariant.Dark;
+    }
+
     public override void OnFrameworkInitializationCompleted()
     {
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)