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

ThemeManager.cs

@@ -0,0 +1,16 @@
+using System;
+using Avalonia;
+
+namespace PLG_Exam;
+
+public static class ThemeManager
+{
+    public static string CurrentBackgroundColor
+    {
+        get
+        {
+            return nameof(Application.RequestedThemeVariant) + "ModeBackground";
+            //return isDarkMode ? "DarkModeBackground" : "LightModeBackground";
+        }
+    }
+}