title: Groupbox gradient background
body: <p>I can't figure out how to make the background og the groupbox fade between almost transparent in the top and almost solid white in the bottom.</p>
  
  <p>I want to set the background in a style like this one:</p>
  
  <pre><code>&lt;ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"&gt;
  
      &lt;!--Groups and borders--&gt;
      &lt;Style x:Key="MainGroupBox" TargetType="{x:Type GroupBox}"&gt;
          &lt;Setter Property="Background" Value="Transparent" /&gt;
          &lt;Setter Property="BorderBrush" Value="Black" /&gt;
          &lt;Setter Property="BorderThickness" Value="1" /&gt;
          &lt;Setter Property="SnapsToDevicePixels" Value="True" /&gt;
      &lt;/Style&gt;
  </code></pre>
  
  <p>I hope you can help me.</p>
  
  <p>PS: I am using C# with WPF 4</p>