如何设置WPF扩展器标头的样式？

<DataGrid>
    <DataGrid.Resources>
       <Style x:Key="ExtenderHeaderStyle" TargetType="{x:Type DataGridColumnHeader}">
           <Setter Property="Background" Value="Blue" />
           <Setter Property="Foreground" Value="White" />
           <Setter Property="FontWeight" Value="Bold" />
           <Setter Property="BorderThickness" Value="1" />
           <Setter Property="BorderBrush" Value="Black" />
        </Style>
    </DataGrid.Resources>
    <DataGrid.Columns>
        <DataGridTextColumn Header="Column 1" HeaderStyle="{StaticResource ExtenderHeaderStyle}" />
        <DataGridTextColumn Header="Column 2" HeaderStyle="{StaticResource ExtenderHeaderStyle}" />
    </DataGrid.Columns>
</DataGrid>