aistudio-wpf-diagram/AIStudio.Wpf.DiagramDesigner/Resources/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>