MaterialDesignTheme.GroupBox.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:converters="clr-namespace:MaterialDesignThemes.Wpf.Converters"
                    xmlns:wpf="clr-namespace:MaterialDesignThemes.Wpf"
                    >

    <Style x:Key="MaterialDesignHeaderedContentControl" TargetType="{x:Type HeaderedContentControl}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type HeaderedContentControl}">
                    <StackPanel>
                        <wpf:ColorZone>
                            <ContentPresenter ContentSource="Header"/>
                        </wpf:ColorZone>
                        <ContentPresenter/>
                    </StackPanel>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    
    <Style x:Key="MaterialDesignGroupBox" TargetType="{x:Type GroupBox}">
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="Foreground" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}, Path=(TextElement.Foreground)}"/>
        <Setter Property="Padding" Value="9"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="SnapsToDevicePixels" Value="true"/>
        <Setter Property="wpf:ShadowAssist.ShadowDepth" Value="Depth0" />
        <Setter Property="wpf:ColorZoneAssist.Mode" Value="PrimaryMid" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type GroupBox}">
                    <Grid>
                        <Border Background="{TemplateBinding Background}" BorderBrush="{Binding Path=Background, ElementName=PART_ColorZone}" BorderThickness="{TemplateBinding BorderThickness}" />
                        <DockPanel Background="{TemplateBinding Background}">
                            <wpf:ColorZone UseLayoutRounding="True" x:Name="PART_ColorZone" DockPanel.Dock="Top" Padding="{TemplateBinding Padding}" 
                                           Effect="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ShadowAssist.ShadowDepth), Converter={x:Static converters:ShadowConverter.Instance}}"
                                           wpf:ShadowAssist.ShadowEdges="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ShadowAssist.ShadowEdges)}"
                                           Mode="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ColorZoneAssist.Mode)}"
                                           wpf:ColorZoneAssist.Background="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ColorZoneAssist.Background)}"
                                           wpf:ColorZoneAssist.Foreground="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ColorZoneAssist.Foreground)}"
                                           SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}">
                                <ContentPresenter ContentSource="Header" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                                  ContentStringFormat="{TemplateBinding HeaderStringFormat}"
                                                  ContentTemplate="{TemplateBinding HeaderTemplate}"
                                                  ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}"/>
                            </wpf:ColorZone>
                            <ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" Margin="{TemplateBinding Padding}"
                                              ContentStringFormat="{TemplateBinding ContentStringFormat}"
                                              ContentTemplate="{TemplateBinding ContentTemplate}"
                                              ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"/>
                        </DockPanel>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style x:Key="MaterialDesignCardGroupBox" TargetType="{x:Type GroupBox}">
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="Foreground" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}, Path=(TextElement.Foreground)}"/>
        <Setter Property="Padding" Value="9"/>
        <Setter Property="BorderThickness" Value="0"/>
        <Setter Property="SnapsToDevicePixels" Value="true"/>
        <Setter Property="wpf:ShadowAssist.ShadowDepth" Value="Depth0" />
        <Setter Property="wpf:ColorZoneAssist.Mode" Value="PrimaryMid" />
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type GroupBox}">
                    <wpf:Card VerticalAlignment="Stretch">
                        <DockPanel Background="{TemplateBinding Background}">
                            <wpf:ColorZone x:Name="PART_ColorZone" DockPanel.Dock="Top" Padding="{TemplateBinding Padding}" 
                                           Effect="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ShadowAssist.ShadowDepth), Converter={x:Static converters:ShadowConverter.Instance}}" 
                                           Mode="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ColorZoneAssist.Mode)}"                                           
                                           wpf:ColorZoneAssist.Background="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ColorZoneAssist.Background)}"
                                           wpf:ColorZoneAssist.Foreground="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(wpf:ColorZoneAssist.Foreground)}">
                                <ContentPresenter ContentSource="Header" RecognizesAccessKey="True" SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}"
                                                  ContentStringFormat="{TemplateBinding HeaderStringFormat}"
                                                  ContentTemplate="{TemplateBinding HeaderTemplate}"
                                                  ContentTemplateSelector="{TemplateBinding HeaderTemplateSelector}"/>
                            </wpf:ColorZone>
                            <ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" Margin="{TemplateBinding Padding}"
                                              ContentStringFormat="{TemplateBinding ContentStringFormat}"
                                              ContentTemplate="{TemplateBinding ContentTemplate}"
                                              ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"/>
                        </DockPanel>
                    </wpf:Card>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>