Hack around the render problem for now.

By applying rotation to path instead of toggle button itself.

Perspex.Direct2D1/Renderer.cs

@@ -144,11 +144,6 @@ namespace Perspex.Direct2D1
                         this.Render(child, context);
                     }
                 }
-
-                if (visual.RenderTransform != null)
-                {
-                    ((RotateTransform)visual.RenderTransform).Angle++;
-                }
             }
         }
     }

Perspex/Themes/Default/TreeViewItemStyle.cs

@@ -48,11 +48,11 @@ namespace Perspex.Themes.Default
                         new Setter(ToggleButton.TemplateProperty, ControlTemplate.Create<ToggleButton>(this.ToggleButtonTemplate)),
                     },
                 },
-                new Style(x => x.OfType<TreeViewItem>().Template().OfType<ToggleButton>().Class("expander").Class(":checked"))
+                new Style(x => x.OfType<TreeViewItem>().Template().OfType<ToggleButton>().Class("expander").Class(":checked").Template().OfType<Path>())
                 {
                     Setters = new[]
                     {
-                        new Setter(ToggleButton.RenderTransformProperty, new RotateTransform(1)),
+                        new Setter(ToggleButton.RenderTransformProperty, new RotateTransform(90)),
                     },
                 },
                 new Style(x => x.OfType<TreeViewItem>().Class(":empty").Template().OfType<ToggleButton>().Class("expander"))
@@ -112,7 +112,6 @@ namespace Perspex.Themes.Default
         {
             return new Border
             {
-                Background = Brushes.Chartreuse,
                 Content = new Path
                 {
                     Fill = Brushes.Black,