SmartThreadPool/STPSLDemo/Groupbox.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:STPSLDemo="clr-namespace:STPSLDemo"
    >
    <Style TargetType="STPSLDemo:Groupbox" >
        <Setter Property="BorderBrush" Value="DarkGray"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="Background" Value="White"/>
        <Setter Property="Padding" Value="6"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="STPSLDemo:Groupbox">
                    <Grid Background="{TemplateBinding Background}">
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="*"/>
                        </Grid.RowDefinitions>
                        <Border BorderThickness="{TemplateBinding BorderThickness}" Grid.Row="1" Grid.RowSpan="2" BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="3" />
                        <ContentPresenter Grid.Row="2" ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Margin="{TemplateBinding Padding}"/>
                        <ContentControl Margin="6,0,0,0" Grid.Row="0" Grid.RowSpan="2">
                            <Border Background="{TemplateBinding Background}">
                                <ContentPresenter Margin="2,0,2,0" ContentTemplate="{TemplateBinding HeaderTemplate}" Content="{TemplateBinding Header}"/>
                            </Border>
                        </ContentControl>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>