Use a concrete type for the host type to avoid #959.

src/Avalonia.Controls/Design.cs

@@ -48,14 +48,14 @@ namespace Avalonia.Controls
         }
         
         public static readonly AttachedProperty<Control> PreviewWithProperty = AvaloniaProperty
-            .RegisterAttached<IStyle, Control>("PreviewWith", typeof (Design));
+            .RegisterAttached<Style, Control>("PreviewWith", typeof (Design));
 
-        public static void SetPreviewWith(IStyle target, Control control)
+        public static void SetPreviewWith(Style target, Control control)
         {
             target.SetValue(PreviewWithProperty, control);
         }
 
-        public static Control GetPreviewWith(IStyle target)
+        public static Control GetPreviewWith(Style target)
         {
             return target.GetValue(PreviewWithProperty);
         }

src/Avalonia.DesignerSupport/DesignWindowLoader.cs

@@ -36,8 +36,7 @@ namespace Avalonia.DesignerSupport
                 var styles = loaded as Styles;
                 if (styles != null)
                 {
-                    var substitute = Design.GetPreviewWith(styles) ??
-                                     styles.Select(Design.GetPreviewWith).FirstOrDefault(s => s != null);
+                    var substitute = styles.OfType<Style>().Select(Design.GetPreviewWith).FirstOrDefault(s => s != null);
                     if (substitute != null)
                     {
                         substitute.Styles.AddRange(styles);

src/Avalonia.Styling/Styling/IStyle.cs

@@ -8,7 +8,7 @@ namespace Avalonia.Styling
     /// <summary>
     /// Defines the interface for styles.
     /// </summary>
-    public interface IStyle : IAvaloniaObject, IResourceNode
+    public interface IStyle : IResourceNode
     {
         /// <summary>
         /// Attaches the style to a control if the style's selector matches.