title: Change background color of header in WPF expander
body: <p>I am trying to change the expander background color. It seems so easy but I can't get it to work.<br/><br/></p>
  
  <pre><code>&lt;Expander Name="expOneDay"&gt;
          &lt;Expander.Header&gt;
              &lt;TextBlock Foreground="CadetBlue" Text="Some Text" HorizontalAlignment="Stretch" /&gt;
          &lt;/Expander.Header&gt;
   ...
  
  &lt;/Expander&gt;&lt;br/&gt;&lt;br/&gt;
  </code></pre>
  
  <p>Why doesn't <code>HorizontalAlignment="Stretch"</code> help? I am trying to bind the width of <code>Header</code> to the width of <code>Expander</code> but the result is not nice looking.</p>