feat: add more templates.

src/Ursa.Themes.Semi/Converters/NavMenuMarginConverter.cs

@@ -0,0 +1,17 @@
+using System.Globalization;
+using Avalonia;
+using Avalonia.Data.Converters;
+
+namespace Ursa.Themes.Semi.Converters;
+
+public class NavMenuMarginConverter: IMultiValueConverter
+{
+    public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (values[0] is double indent && values[1] is int level)
+        {
+            return new Thickness(indent * level, 0, 0, 0);
+        }
+        return AvaloniaProperty.UnsetValue;
+    }
+}