feat: Add Badge and Badge demo.

demo/Ursa.Demo/Views/MainWindow.axaml.cs

@@ -1,4 +1,7 @@
+using Avalonia;
 using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Styling;
 
 namespace Ursa.Demo.Views;
 
@@ -8,4 +11,14 @@ public partial class MainWindow : Window
     {
         InitializeComponent();
     }
+
+    private void ToggleButton_OnIsCheckedChanged(object? sender, RoutedEventArgs e)
+    {
+        var app = Application.Current;
+        if (app is not null)
+        {
+            var theme = app.ActualThemeVariant;
+            app.RequestedThemeVariant = theme == ThemeVariant.Dark ? ThemeVariant.Light : ThemeVariant.Dark;
+        }
+    }
 }