feat: Add theme.

demo/Ursa.Demo/ViewModels/ScrollToButtonDemoViewModel.cs

@@ -0,0 +1,15 @@
+using System.Collections.ObjectModel;
+using System.Linq;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Ursa.Demo.ViewModels;
+
+public class ScrollToButtonDemoViewModel: ObservableObject
+{
+    public ObservableCollection<string> Items { get; set; }
+
+    public ScrollToButtonDemoViewModel()
+    {
+        Items = new ObservableCollection<string>(Enumerable.Range(0, 1000).Select(a => "Item " + a));
+    }
+}