title: Base Expander Style, override header color
body: <p>Is it possible to create some sort of base expander style and override the background color of the header in a derived style?
  In my application I'm using expanders a lot and I would like to change the background color of the header. I know I could just copy&amp;paste my style and edit the color, but it would be nicer to just create a new style based on the "base style" and setting the header's background color.
  But I do not know how to access this color.
  It's the color of this line: below I'd like to change (the border in the header): Border Name="border"... I cannot access "border" in the setter of the derived style...</p>
  
  <p>This is my (base) style:</p>
  
  <pre><code>&lt;Style TargetType="Expander" x:Key="ExpanderStyle"&gt;
      &lt;Setter Property="Background" Value="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}"/&gt;
      &lt;Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextColor}}"/&gt;
      &lt;Setter Property="Template"&gt;
          &lt;Setter.Value&gt;
              &lt;!-- Control template for expander --&gt;
              &lt;ControlTemplate TargetType="Expander" x:Name="exp"&gt;
                  &lt;Grid&gt;
                      &lt;Grid.RowDefinitions&gt;
                          &lt;RowDefinition Height="Auto"/&gt;
                          &lt;RowDefinition Name="ContentRow" Height="0"/&gt;
                      &lt;/Grid.RowDefinitions&gt;
                      &lt;Border Name="border" Grid.Row="0" Background="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}" BorderThickness="1" CornerRadius="4,4,0,0" &gt;
                          &lt;Grid&gt;
                              &lt;Grid.ColumnDefinitions&gt;
                                  &lt;ColumnDefinition Width="*" /&gt;
                                  &lt;ColumnDefinition Width="20" /&gt;
                              &lt;/Grid.ColumnDefinitions&gt;
                              &lt;ToggleButton x:Name="tb" FontFamily="Marlett" FontSize="9.75" Background="{DynamicResource {x:Static SystemColors.ControlDarkBrushKey}}" Foreground="Black" Grid.Column="1" Content="u" IsChecked="{Binding Path=IsExpanded,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}" /&gt;
                              &lt;ContentPresenter x:Name="HeaderContent" Grid.Column="0" Margin="4" ContentSource="Header" RecognizesAccessKey="True" /&gt;
                          &lt;/Grid&gt;
                      &lt;/Border&gt;
                      &lt;Border x:Name="Content" Grid.Row="1" BorderThickness="1,0,1,1" CornerRadius="0,0,4,4" &gt;
                          &lt;ContentPresenter Margin="4" /&gt;
                      &lt;/Border&gt;
                  &lt;/Grid&gt;
                  &lt;ControlTemplate.Triggers&gt;
                      &lt;Trigger Property="IsExpanded" Value="True"&gt;
                          &lt;Setter TargetName="ContentRow" Property="Height" Value="{Binding ElementName=Content,Path=Height}" /&gt;
                          &lt;Setter Property="Content" TargetName="tb" Value="t"&gt;&lt;/Setter&gt;
                      &lt;/Trigger&gt;
                  &lt;/ControlTemplate.Triggers&gt;
              &lt;/ControlTemplate&gt;
          &lt;/Setter.Value&gt;
      &lt;/Setter&gt;
  &lt;/Style&gt;
  </code></pre>
  
  <p>I would like to do something like this:</p>
  
  <pre><code>&lt;Style x:Key="ExpanderStyleRed" BasedOn="{StaticResource ExpanderStyle}" TargetType="Expander"&gt;
              &lt;Setter Property="???" Value="Red"/&gt;
  &lt;Style&gt;
  </code></pre>