我用如下代码设置表头样式

<my:DataGridTemplateColumn Header="照片" >
                                    <my:DataGridTemplateColumn.HeadTemplate >

                                        <DataTemplate>
                                            <StackPanel>
                                                <TextBlock Background="#666666"></TextBlock >
                                            </StackPanel>
                                          

                                        </DataTemplate>
                                    </my:DataGridTemplateColumn.HeadTemplate >
                                   
                                </my:DataGridTemplateColumn>

结果列头只有中间一部分颜色改变，设置了高和宽之后，列头总是有一圈灰色的默认背景，怎么才能设置列头颜色呢？