Can apply control theme to derived types.

src/Avalonia.Base/Styling/ControlTheme.cs

@@ -37,7 +37,7 @@ namespace Avalonia.Styling
 
             var result = BasedOn?.TryAttach(target, host) ?? SelectorMatchResult.NeverThisType;
 
-            if (HasSettersOrAnimations && target.StyleKey == TargetType)
+            if (HasSettersOrAnimations && TargetType.IsAssignableFrom(target.StyleKey))
             {
                 Attach(target, null);
                 result = SelectorMatchResult.AlwaysThisType;

src/Avalonia.Base/Styling/NestingSelector.cs

@@ -23,7 +23,7 @@ namespace Avalonia.Styling
             {
                 if (theme.TargetType is null)
                     throw new InvalidOperationException("ControlTheme has no TargetType.");
-                return control.StyleKey == theme.TargetType ?
+                return theme.TargetType.IsAssignableFrom(control.StyleKey) ?
                     SelectorMatch.AlwaysThisType :
                     SelectorMatch.NeverThisType;
             }

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

@@ -34,6 +34,27 @@ public class StyledElementTests_Theming
             Assert.Equal(Brushes.Green, border.Background);
         }
 
+        [Fact]
+        public void Theme_Is_Applied_To_Derived_Class_When_Attached_To_Logical_Tree()
+        {
+            using var app = UnitTestApplication.Start(TestServices.RealStyler);
+            var target = new DerivedThemedControl
+            {
+                Theme = CreateTheme(),
+            };
+
+            Assert.Null(target.Template);
+
+            var root = CreateRoot(target);
+            Assert.NotNull(target.Template);
+
+            var border = Assert.IsType<Border>(target.VisualChild);
+            Assert.Equal(Brushes.Red, border.Background);
+
+            target.Classes.Add("foo");
+            Assert.Equal(Brushes.Green, border.Background);
+        }
+
         [Fact]
         public void Theme_Is_Detached_When_Theme_Property_Cleared()
         {
@@ -252,4 +273,8 @@ public class StyledElementTests_Theming
     {
         public IVisual? VisualChild => VisualChildren?.SingleOrDefault();
     }
+
+    private class DerivedThemedControl : ThemedControl
+    {
+    }
 }