title: WPF Custom Control based on TabItem and Themes
body: <p>I have a Generic.xaml theme created that sets styles for all common controls, including TabItem
  This them is applying ok to all controls in the application</p>
  
  <p>When I create a TabItem control and display it, it gets the Theme OK.</p>
  
  <pre><code>TabItem t = new TabItem();
   t.Header = "Normal";
   MainContentControl.Items.Add(t);
  </code></pre>
  
  <p>However when i create a Custom Control based on TabItem</p>
  
  <pre><code> public partial class ClosableTab : TabItem
  </code></pre>
  
  <p>and display it</p>
  
  <pre><code>ProActive.LocalControls.ClosableTab ct = new ProActive.LocalControls.ClosableTab();
    ct.Header = "COMP";
    MainContentControl.Items.Add(ct);
  </code></pre>
  
  <p>The theme is ignored.</p>
  
  <p>I have tried over writing its defaultstyle using</p>
  
  <pre><code> DefaultStyleKeyProperty.OverrideMetadata(typeof(ClosableTab), new FrameworkPropertyMetadata(typeof(TabItem)));
  </code></pre>
  
  <p>Why if my custom control is based on a TabItem is the theme not also applying to it?</p>