WPF，如何调整分组框透明度

<Window x:Class="WpfApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="WPF GroupBox Opacity" Height="450" Width="800">
    <Grid>
        <GroupBox Header="Sample GroupBox" Opacity="0.5">
            <TextBlock Text="This is a sample groupbox with reduced opacity."/>
        </GroupBox>
    </Grid>
</Window>