feat: improve styling.

src/Ursa/Converters/SelectionBoxTemplateConverter.cs

@@ -0,0 +1,16 @@
+using System.Globalization;
+using Avalonia.Controls.Templates;
+using Avalonia.Data.Converters;
+
+namespace Ursa.Converters;
+
+public class SelectionBoxTemplateConverter: IMultiValueConverter
+{
+    public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
+    {
+        var selectedItemTemplate = values.Count > 0 ? values[0] as IDataTemplate : null;
+        if (selectedItemTemplate is not null) return selectedItemTemplate;
+        var itemTemplate = values.Count > 1 ? values[1] as IDataTemplate : null;
+        return itemTemplate;
+    }
+}