PivotItem的边框刷没有在我的UWP应用程序中设置

Question

PivotItem的边框画笔未在我的UWP应用中设置。下面是我的xaml代码。请帮忙..

<Pivot x:Name="DocTab" Title="" VerticalAlignment="Top" Height="980"  Margin="0,50" BorderBrush="Black" BorderThickness="1" >
    <PivotItem Header="Document 1" Margin="40,30"  >
    </PivotItem>
    <PivotItem Header="Document 2"  VerticalAlignment="Top" Height="940">
    </PivotItem>
</Pivot>