feat: update to finish functionality, start to build theme.

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

@@ -0,0 +1,23 @@
+using System.Globalization;
+using Avalonia;
+using Avalonia.Data.Converters;
+
+namespace Ursa.Themes.Semi.Converters;
+
+public class NavigationMenuItemLevelToMarginConverter: IValueConverter
+{
+    public int Indent { get; set; }
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (value is int i)
+        {
+            return new Thickness((i-1) * Indent, 0, 0, 0);
+        }
+        return new Thickness();
+    }
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        throw new NotImplementedException();
+    }
+}