背景颜色与边框背景颜色不同时，为WPF TextBlock设置左上角和右上角不起作用

<Grid Background="{边框背景色}">
    <Rectangle Width="10" Height="10" Fill="{左上角背景色}" HorizontalAlignment="Left" VerticalAlignment="Top"/>
    <Rectangle Width="10" Height="10" Fill="{右上角背景色}" HorizontalAlignment="Right" VerticalAlignment="Top"/>
    <TextBlock Text="{文本内容}" />
</Grid>

<Style x:Key="CustomTextBlockStyle" TargetType="TextBlock">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="TextBlock">
                <Grid Background="{边框背景色}">
                    <Rectangle x:Name="TopLeftCorner" Width="10" Height="10" Fill="{左上角背景色}" HorizontalAlignment="Left" VerticalAlignment="Top"/>
                    <Rectangle x:Name="TopRightCorner" Width="10" Height="10" Fill="{右上角背景色}" HorizontalAlignment="Right" VerticalAlignment="Top"/>
                    <ContentPresenter/>
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter TargetName="TopLeftCorner" Property="Fill" Value="{鼠标悬停时左上角背景色}"/>
                        <Setter TargetName="TopRightCorner" Property="Fill" Value="{鼠标悬停时右上角背景色}"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<TextBlock Style="{StaticResource CustomTextBlockStyle}" Text="{文本内容}" />