去除WPF中GroupBox边框的白边

<Window.Resources>
<BorderGapMaskConverter x:Key="BorderGapMaskConverter"/>
<Style x:Key="GroupboxWithoutWhiteBorder" TargetType="{x:Type GroupBox}">
<Setter Property="BorderBrush" Value="#D5DFE5" />
<Setter Property="BorderThickness" Value="1" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type GroupBox}">
<Grid SnapsToDevicePixels="true">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="6" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width="6" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
<RowDefinition Height="6" />
</Grid.RowDefinitions>
<Border CornerRadius="4" Grid.Row="1" Grid.RowSpan="3" Grid.Column="0" Grid.ColumnSpan="4" BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="Transparent" Background="{TemplateBinding Background}" />
<Border Name="Header" Padding="3,1,3,0" Grid.Row="0" Grid.RowSpan="2" Grid.Column="1">
<ContentPresenter ContentSource="Header" RecognizesAccessKey="true" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
</Border>
<ContentPresenter Grid.Row="2" Grid.Column="1" Grid.ColumnSpan="2" Margin="{TemplateBinding Padding}" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" />
<Border Grid.Row="1" Grid.RowSpan="3" Grid.ColumnSpan="4"  BorderThickness="{TemplateBinding BorderThickness}" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="3">
<Border.OpacityMask>
<MultiBinding Converter="{StaticResource BorderGapMaskConverter}" ConverterParameter="7">
<Binding ElementName="Header" Path="ActualWidth" />
<Binding Path="ActualWidth" RelativeSource="{RelativeSource Self}" />
<Binding Path="ActualHeight" RelativeSource="{RelativeSource Self}" />
</MultiBinding>
</Border.OpacityMask>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
</Window.Resources>

<Grid Background="Green">
<GroupBox Header="WPF边框白边" Margin="20" Style="{StaticResource GroupboxWithoutWhiteBorder}"/>
</Grid>