Make StyledElement use theme variants again

src/Avalonia.Base/StyledElement.cs

@@ -28,7 +28,7 @@ namespace Avalonia
     public class StyledElement : Animatable, 
         IDataContextProvider, 
         ILogical,
-        IResourceHost,
+        IThemeVariantHost,
         IStyleHost,
         IStyleable,
         ISetLogicalParent,
@@ -143,6 +143,9 @@ namespace Avalonia
         /// </summary>
         public event EventHandler<ResourcesChangedEventArgs>? ResourcesChanged;
 
+        /// <inheritdoc />
+        public event EventHandler? ActualThemeVariantChanged;
+        
         /// <summary>
         /// Gets or sets the name of the styled element.
         /// </summary>
@@ -299,6 +302,9 @@ namespace Avalonia
         /// </summary>
         public StyledElement? Parent { get; private set; }
 
+        /// <inheritdoc />
+        public ThemeVariant ActualThemeVariant => GetValue(ThemeVariant.ActualThemeVariantProperty);
+        
         /// <summary>
         /// Gets the styled element's logical parent.
         /// </summary>
@@ -618,7 +624,20 @@ namespace Avalonia
             base.OnPropertyChanged(change);
 
             if (change.Property == ThemeProperty)
+            {
                 OnControlThemeChanged();
+            }
+            else if (change.Property == ThemeVariant.RequestedThemeVariantProperty)
+            {
+                if (change.GetNewValue<ThemeVariant>() is {} themeVariant && themeVariant != ThemeVariant.Default)
+                    SetValue(ThemeVariant.ActualThemeVariantProperty, themeVariant);
+                else
+                    ClearValue(ThemeVariant.ActualThemeVariantProperty);
+            }
+            else if (change.Property == ThemeVariant.ActualThemeVariantProperty)
+            {
+                ActualThemeVariantChanged?.Invoke(this, EventArgs.Empty);
+            }
         }
 
         private protected virtual void OnControlThemeChanged()

src/Avalonia.Base/Styling/ThemeVariant.cs

@@ -11,6 +11,21 @@ namespace Avalonia.Styling;
 [TypeConverter(typeof(ThemeVariantTypeConverter))]
 public sealed record ThemeVariant
 {
+    /// <summary>
+    /// Defines the ActualThemeVariant property.
+    /// </summary>
+    internal static readonly StyledProperty<ThemeVariant> ActualThemeVariantProperty =
+        AvaloniaProperty.Register<StyledElement, ThemeVariant>(
+            "ActualThemeVariant",
+            inherits: true);
+
+    /// <summary>
+    /// Defines the RequestedThemeVariant property.
+    /// </summary>
+    internal static readonly StyledProperty<ThemeVariant?> RequestedThemeVariantProperty =
+        AvaloniaProperty.Register<StyledElement, ThemeVariant?>(
+            "RequestedThemeVariant", defaultValue: Default);
+    
     /// <summary>
     /// Creates a new instance of the <see cref="ThemeVariant"/>
     /// </summary>

src/Avalonia.Controls/Control.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Controls
     /// - A <see cref="Tag"/> property to allow user-defined data to be attached to the control.
     /// - <see cref="ContextRequestedEvent"/> and other context menu related members.
     /// </remarks>
-    public class Control : InputElement, IDataTemplateHost, INamed, IVisualBrushInitialize, ISetterValue, IThemeVariantHost
+    public class Control : InputElement, IDataTemplateHost, INamed, IVisualBrushInitialize, ISetterValue
     {
         /// <summary>
         /// Defines the <see cref="FocusAdorner"/> property.
@@ -162,11 +162,6 @@ namespace Avalonia.Controls
             set => SetValue(TagProperty, value);
         }
         
-        /// <inheritdoc />
-        public ThemeVariant ActualThemeVariant => GetValue(ThemeVariantScope.ActualThemeVariantProperty);
-
-        public event EventHandler? ActualThemeVariantChanged;
-
         /// <summary>
         /// Occurs when the user has completed a context input gesture, such as a right-click.
         /// </summary>
@@ -531,17 +526,6 @@ namespace Avalonia.Controls
                     RaiseEvent(sizeChangedEventArgs);
                 }
             }
-            else if (change.Property == ThemeVariantScope.RequestedThemeVariantProperty)
-            {
-                if (change.GetNewValue<ThemeVariant>() is {} themeVariant && themeVariant != ThemeVariant.Default)
-                    SetValue(ThemeVariantScope.ActualThemeVariantProperty, themeVariant);
-                else
-                    ClearValue(ThemeVariantScope.ActualThemeVariantProperty);
-            }
-            else if (change.Property == ThemeVariantScope.ActualThemeVariantProperty)
-            {
-                ActualThemeVariantChanged?.Invoke(this, EventArgs.Empty);
-            }
         }
     }
 }

src/Avalonia.Controls/ThemeVariantScope.cs

@@ -7,21 +7,13 @@ namespace Avalonia.Controls
     /// </summary>
     public class ThemeVariantScope : Decorator
     {
-        /// <summary>
-        /// Defines the <see cref="Control.ActualThemeVariant"/> property.
-        /// </summary>
+        /// <inheritdoc cref="ThemeVariant.ActualThemeVariantProperty" />
         public static readonly StyledProperty<ThemeVariant> ActualThemeVariantProperty =
-            AvaloniaProperty.Register<ThemeVariantScope, ThemeVariant>(
-                nameof(ActualThemeVariant),
-                inherits: true);
+            ThemeVariant.ActualThemeVariantProperty.AddOwner<ThemeVariantScope>();
 
-        /// <summary>
-        /// Defines the <see cref="RequestedThemeVariant"/> property.
-        /// </summary>
+        /// <inheritdoc cref="ThemeVariant.RequestedThemeVariantProperty" />
         public static readonly StyledProperty<ThemeVariant?> RequestedThemeVariantProperty =
-            AvaloniaProperty.Register<ThemeVariantScope, ThemeVariant?>(
-                nameof(RequestedThemeVariant),
-                defaultValue: ThemeVariant.Default);
+            ThemeVariant.RequestedThemeVariantProperty.AddOwner<ThemeVariantScope>();
 
         /// <summary>
         /// Gets or sets the UI theme variant that is used by the control (and its child elements) for resource determination.