fix: support WASM.

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

@@ -0,0 +1,24 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Interactivity;
+using Avalonia.Styling;
+
+namespace Ursa.Demo.Views;
+
+public partial class MainView : UserControl
+{
+    public MainView()
+    {
+        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;
+        }
+    }
+}