关于 silverlight DataGridColumnHeader 的背景颜色不能改变 急 帮帮忙啊解决方法

关于 silverlight DataGridColumnHeader 的背景颜色不能改变 急 帮帮忙啊
我是这样写的

命名空间
xmlns:Primitives="clr-namespace:System.Windows.Controls.Primitives;assembly=System.Windows.Controls.Data" 

静态资源
        <UserControl.Resources>
             <Style x:Key="gridheader" TargetType="Primitives:DataGridColumnHeader">
<Setter Property="Background">
                <Setter.Value>
                    <LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
                        <GradientStop Color="White" Offset="0"/>
                        <GradientStop Color="Yellow" Offset="0.5"/>
                        <GradientStop Color="White" Offset="1"/>
                    </LinearGradientBrush>
                </Setter.Value>
</Setter>
            </Style>
        </UserControl.Resources>

datagrid绑定了gridheader
<data:DataGridTextColumn Header="名称" Width="190" IsReadOnly="True"  Binding="{Binding DevName, 

Mode=OneWay}" HeaderStyle="{StaticResource gridheader}"/>

为什么datagrid 的header的背景颜色没有改变呢？
大家给看看  怎么能改变header的背景颜色呢？
------解决方案--------------------
header的背景颜色这么改是改不掉的，要在模板里改，可以参考下面这个样式修改：

<Style x:Key="gridheader" TargetType="sdk:DataGridColumnHeader">

        <Setter Property="Foreground" Value="#FFB5B5B5" />

        <Setter Property="HorizontalContentAlignment" Value="Center" />

        <Setter Property="VerticalContentAlignment" Value="Center" />

        <Setter Property="FontSize" Value="10.5" />

        <Setter Property="FontWeight" Value="Bold" />

        <Setter Property="IsTabStop" Value="False" />

        <Setter Property="SeparatorBrush" Value="#FFDFE3E6" />

        <Setter Property="Padding" Value="4,4,5,4" />

        <Setter Property="Template">

          <Setter.Value>

            <ControlTemplate TargetType="sdk:DataGridColumnHeader">

              <Grid Name="Root">

                <VisualStateManager.VisualStateGroups>