diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml
index 1270dbaa62..d2b31fdb20 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml
@@ -3,8 +3,7 @@
         xmlns:views="clr-namespace:Avalonia.Diagnostics.Views"
         xmlns:diag="clr-namespace:Avalonia.Diagnostics"
         Title="Avalonia DevTools"
-        x:Class="Avalonia.Diagnostics.Views.MainWindow"
-        Theme="{StaticResource {x:Type Window}}">
+        x:Class="Avalonia.Diagnostics.Views.MainWindow">
   <Window.DataTemplates>
     <diag:ViewLocator/>
   </Window.DataTemplates>
diff --git a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
index c81997f2cb..e6e630112b 100644
--- a/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
+++ b/src/Avalonia.Diagnostics/Diagnostics/Views/MainWindow.xaml.cs
@@ -28,6 +28,11 @@ namespace Avalonia.Diagnostics.Views
         {
             InitializeComponent();
 
+            // Apply the SimpleTheme.Window theme; this must be done after the XAML is parsed as
+            // the theme is included in the MainWindow's XAML.
+            if (Theme is null && this.FindResource(typeof(Window)) is ControlTheme windowTheme)
+                Theme = windowTheme;
+
             _keySubscription = InputManager.Instance?.Process
                 .OfType<RawKeyEventArgs>()
                 .Where(x => x.Type == RawKeyEventType.KeyDown)
diff --git a/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/StaticResourceExtension.cs b/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/StaticResourceExtension.cs
index d462a2210e..f28f7bc626 100644
--- a/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/StaticResourceExtension.cs
+++ b/src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/StaticResourceExtension.cs
@@ -56,7 +56,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
                 // We need to implement compile-time merging of resource dictionaries and this
                 // hack can be removed.
                 if (previousWasControlTheme &&
-                    parent is ResourceDictionary hack &&
+                    parent is IResourceProvider hack &&
                     hack.Owner?.GetType().FullName == "Avalonia.Diagnostics.Views.MainWindow" &&
                     hack.Owner.TryGetResource(ResourceKey, out value))
                 {