title: Update style on element when ValidationError has occured, with an overriden template
body: <p>I've overriden the <code>Template</code> of <code>TextBox</code> to give it a rounded border:</p>
  
  <pre><code>&lt;Style TargetType="TextBox"&gt;
      &lt;Setter Property="b:TextBoxBehaviours.SelectAllTextOnFocus" Value="True" /&gt;
      &lt;Setter Property="Width" Value="75" /&gt;
      &lt;Setter Property="Margin" Value="2" /&gt;
      &lt;Setter Property="Template"&gt;
          &lt;Setter.Value&gt;
              &lt;ControlTemplate&gt;
                  &lt;Border x:Name="brdTxtBx" BorderThickness="2" CornerRadius="3" BorderBrush="{x:Static SystemColors.ActiveBorderBrush}"&gt;
                      &lt;ScrollViewer x:Name="PART_ContentHost" /&gt;
                  &lt;/Border&gt;
              &lt;/ControlTemplate&gt;
          &lt;/Setter.Value&gt;
      &lt;/Setter&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p>What I'm trying to achieve is that when the <code>TextBox</code> gets a validation error, the <code>Border</code> should change colour to a different one (e.g. Red). I've tried putting a <code>DataTrigger</code> in the <code>Border</code> style, but that doesn't seem to work.</p>
  
  <pre><code>&lt;Border x:Name="brdTxtBx" BorderThickness="2" CornerRadius="3" BorderBrush="{x:Static SystemColors.ActiveBorderBrush}"&gt;
      &lt;Border.Style&gt;
          &lt;Style TargetType="Border"&gt;
              &lt;Style.Triggers&gt;
                  &lt;DataTrigger Binding="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=TextBox}, Path=Validaton.HasError}" Value="True"&gt;
                      &lt;Setter Property="BorderBrush" Value="Red" /&gt;
                  &lt;/DataTrigger&gt;
              &lt;/Style.Triggers&gt;
          &lt;/Style&gt;
      &lt;/Border.Style&gt;
      &lt;ScrollViewer x:Name="PART_ContentHost" /&gt;
  &lt;/Border&gt;
  </code></pre>
  
  <p>I've tried changing the <code>Mode</code> on the binding to see if that works, but doesn't seem to have any affect. I've also changed the <code>DataBinding</code> path to bind to another property (i.e. the <code>Text</code> value), to see if that triggers it, so I think it's more my understanding of <code>DataTrigger</code>s in <code>ControlTemplate</code>s isn't quite right?</p>
  
  <p>Am I approaching this incorrectly, or just slightly off?.</p>