Feature: BrushRoundConverter supports ImmutableSolidColorBrush objects

These are used for the named colors in Avalonia

Material.Styles/Converters/BrushRoundConverter.cs

@@ -16,7 +16,7 @@ public class BrushRoundConverter : IValueConverter
 
         public object Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
-            if (value is not SolidColorBrush solidColorBrush)
+            if (value is not ISolidColorBrush solidColorBrush)
                 return BindingOperations.DoNothing;
 
             var color = solidColorBrush.Color;