Fix resolving dynamic resource in Application.

Instead of looking for an `IStyledElement` as an anchor, look for an `IResourceHost` because `Application` doesn't implement `IStyledElement` but it does implement `IResourceHost` and this interface has everything we need.

src/Avalonia.Styling/Controls/ResourceNodeExtensions.cs

@@ -60,7 +60,7 @@ namespace Avalonia.Controls
         }
 
         public static IObservable<object?> GetResourceObservable(
-            this IStyledElement control,
+            this IResourceHost control,
             object key,
             Func<object?, object?>? converter = null)
         {
@@ -83,11 +83,11 @@ namespace Avalonia.Controls
 
         private class ResourceObservable : LightweightObservableBase<object?>
         {
-            private readonly IStyledElement _target;
+            private readonly IResourceHost _target;
             private readonly object _key;
             private readonly Func<object?, object?>? _converter;
 
-            public ResourceObservable(IStyledElement target, object key, Func<object?, object?>? converter)
+            public ResourceObservable(IResourceHost target, object key, Func<object?, object?>? converter)
             {
                 _target = target;
                 _key = key;

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/DynamicResourceExtension.cs

@@ -10,8 +10,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 {
     public class DynamicResourceExtension : IBinding
     {
-        private IStyledElement? _anchor;
-        private IResourceProvider? _resourceProvider;
+        private object? _anchor;
 
         public DynamicResourceExtension()
         {
@@ -30,12 +29,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
 
             if (!(provideTarget.TargetObject is IStyledElement))
             {
-                _anchor = serviceProvider.GetFirstParent<IStyledElement>();
-
-                if (_anchor is null)
-                {
-                    _resourceProvider = serviceProvider.GetFirstParent<IResourceProvider>();
-                }
+                _anchor = serviceProvider.GetFirstParent<IStyledElement>() ??
+                    serviceProvider.GetFirstParent<IResourceProvider>() ??
+                    (object?)serviceProvider.GetFirstParent<IResourceHost>();
             }
 
             return this;
@@ -52,16 +48,16 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
                 return null;
             }
 
-            var control = target as IStyledElement ?? _anchor as IStyledElement;
+            var control = target as IResourceHost ?? _anchor as IResourceHost;
 
             if (control != null)
             {
                 var source = control.GetResourceObservable(ResourceKey, GetConverter(targetProperty));
                 return InstancedBinding.OneWay(source);
             }
-            else if (_resourceProvider is object)
+            else if (_anchor is IResourceProvider resourceProvider)
             {
-                var source = _resourceProvider.GetResourceObservable(ResourceKey, GetConverter(targetProperty));
+                var source = resourceProvider.GetResourceObservable(ResourceKey, GetConverter(targetProperty));
                 return InstancedBinding.OneWay(source);
             }