Added unit test for templated parent theme changed loop.

tests/Avalonia.Base.UnitTests/Styling/StyledElementTests_Theming.cs

@@ -569,6 +569,46 @@ public class StyledElementTests_Theming
             Assert.Equal(Brushes.Green, border.Background);
         }
 
+        [Fact]
+        public void TemplatedParent_Theme_Change_Applies_Recursively_To_VisualChildren()
+        {
+            var theme = CreateDerivedTheme();
+            var target = CreateTarget();
+
+            Assert.Null(target.Theme);
+            Assert.Null(target.Template);
+
+            var root = CreateRoot(target, theme.BasedOn);
+
+            Assert.NotNull(target.Theme);
+            Assert.NotNull(target.Template);
+
+            root.Styles.Add(new Style(x => x.OfType<ThemedControl>().Class("foo"))
+            {
+                Setters = { new Setter(StyledElement.ThemeProperty, theme) }
+            });
+
+            root.LayoutManager.ExecuteLayoutPass();
+
+            var border = Assert.IsType<Border>(target.VisualChild);
+            var inner = Assert.IsType<Border>(border.Child);
+
+            Assert.Equal(Brushes.Red, border.Background);
+            Assert.Equal(Brushes.Red, inner.Background);
+
+            Assert.Equal(null, inner.BorderBrush);
+            Assert.Equal(null, inner.BorderBrush);
+
+            target.Classes.Add("foo");
+            root.LayoutManager.ExecuteLayoutPass();
+
+            Assert.Equal(Brushes.Green, border.Background);
+            Assert.Equal(Brushes.Green, inner.Background);
+
+            Assert.Equal(Brushes.Cyan, inner.BorderBrush);
+            Assert.Equal(Brushes.Cyan, inner.BorderBrush);
+        }
+
         private static ThemedControl CreateTarget()
         {
             return new ThemedControl();
@@ -595,7 +635,8 @@ public class StyledElementTests_Theming
 
     private static ControlTheme CreateTheme(string tag = "theme")
     {
-        var template = new FuncControlTemplate<ThemedControl>((o, n) => new Border());
+        var template = new FuncControlTemplate<ThemedControl>(
+            (o, n) => new Border() { Child = new Border() });
 
         return new ControlTheme
         {