aistudio-wpf-diagram/AIStudio.Wpf.DiagramDesigner/Styles/GroupBox.xaml

<ResourceDictionary 
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Shared.xaml" />
    </ResourceDictionary.MergedDictionaries>

    <!-- SimpleStyles: GroupBox -->
    <Style TargetType="GroupBox">
        <Setter Property="FontFamily" Value="SegoeUI"/>
        <Setter Property="FontSize" Value="12"/>
        <Setter Property="Foreground" Value="#4C4C4C"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="GroupBox">
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="Auto"/>
                            <RowDefinition Height="*"/>
                        </Grid.RowDefinitions>
                        <Border 
              Grid.Row="0" 
              Background="{StaticResource LightBrush}"
              BorderBrush="{StaticResource NormalBorderBrush}"
              BorderThickness="1" 
              CornerRadius="2,2,0,0" >
                            <ContentPresenter 
                Margin="4" 
                ContentSource="Header" 
                RecognizesAccessKey="True" />
                        </Border>
                        <Border 
              Grid.Row="1" 
              Background="{TemplateBinding Background}"
              BorderBrush="{StaticResource SolidBorderBrush}" 
              BorderThickness="1,0,1,1" 
              CornerRadius="0,0,2,2" >
                            <ContentPresenter 
                Margin="4" />
                        </Border>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>