The root of a control template has to be Control.

Perspex.Windows/Window.cs

@@ -80,7 +80,7 @@ namespace Perspex.Windows
             UnmanagedMethods.ShowWindow(this.Handle, 4);
         }
 
-        private Visual DefaultTemplate(Window c)
+        private Control DefaultTemplate(Window c)
         {
             Border border = new Border();
             border.Background = new Perspex.Media.SolidColorBrush(0xffffffff);

Perspex/ControlTemplate.cs

@@ -5,18 +5,18 @@
 
     public class ControlTemplate
     {
-        public ControlTemplate(Func<TemplatedControl, Visual> build)
+        public ControlTemplate(Func<TemplatedControl, Control> build)
         {
             this.Build = build;
         }
 
-        public Func<TemplatedControl, Visual> Build
+        public Func<TemplatedControl, Control> Build
         {
             get;
             private set;
         }
 
-        public static ControlTemplate Create<TControl>(Func<TControl, Visual> build)
+        public static ControlTemplate Create<TControl>(Func<TControl, Control> build)
             where TControl : TemplatedControl
         {
             return new ControlTemplate(c => build((TControl)c));

Perspex/Themes/Default/ButtonStyle.cs

@@ -45,7 +45,7 @@ namespace Perspex.Themes.Default
             });
         }
 
-        private Visual Template(Button control)
+        private Control Template(Button control)
         {
             Border border = new Border();
             border.SetValue(Border.BackgroundProperty, control.GetObservable(Button.BackgroundProperty));