Semi.Avalonia/src/Semi.Avalonia/Controls/HeaderedContentControl.axaml

<ResourceDictionary
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:CompileBindings="True">
    <ControlTheme x:Key="{x:Type HeaderedContentControl}" TargetType="HeaderedContentControl">
        <Setter Property="Padding" Value="3" />
        <Setter Property="Template">
            <ControlTemplate>
                <Border
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderThickness="{TemplateBinding BorderThickness}"
                    CornerRadius="{TemplateBinding CornerRadius}"
                    Background="{TemplateBinding Background}"
                    Padding="{TemplateBinding Padding}">
                    <Grid RowDefinitions="auto *">
                        <ContentPresenter
                            Name="PART_HeaderPresenter"
                            Content="{TemplateBinding Header}"
                            RecognizesAccessKey="True"
                            ContentTemplate="{TemplateBinding HeaderTemplate}"
                            HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                            VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                            FontSize="{TemplateBinding FontSize}"
                            FontWeight="{TemplateBinding FontWeight}"
                            FontFamily="{TemplateBinding FontFamily}"
                            FontStyle="{TemplateBinding FontStyle}"
                            Grid.Row="0" />
                        <ContentPresenter
                            Name="PART_ContentPresenter"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            Content="{TemplateBinding Content}"
                            RecognizesAccessKey="True"
                            VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                            HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                            FontSize="{TemplateBinding FontSize}"
                            FontWeight="{TemplateBinding FontWeight}"
                            FontFamily="{TemplateBinding FontFamily}"
                            FontStyle="{TemplateBinding FontStyle}"
                            Grid.Row="1" />
                    </Grid>
                </Border>
            </ControlTemplate>
        </Setter>
    </ControlTheme>

    <ControlTheme x:Key="GroupBox" TargetType="HeaderedContentControl">
        <Setter Property="Background" Value="{DynamicResource HeaderedContentControlDefaultBackground}" />
        <Setter Property="BorderThickness" Value="1" />
        <Setter Property="BorderBrush" Value="{DynamicResource HeaderedContentControlDefaultBorderBrush}" />
        <Setter Property="CornerRadius" Value="{DynamicResource HeaderedContentControlCornerRadius}" />
        <Setter Property="HorizontalAlignment" Value="Stretch" />
        <Setter Property="HorizontalContentAlignment" Value="Left" />
        <Setter Property="VerticalContentAlignment" Value="Stretch" />
        <Setter Property="Template">
            <ControlTemplate TargetType="HeaderedContentControl">
                <Border
                    HorizontalAlignment="Stretch"
                    VerticalAlignment="Stretch"
                    Width="{TemplateBinding Width}"
                    Height="{TemplateBinding Height}"
                    Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    CornerRadius="{TemplateBinding CornerRadius}"
                    BorderThickness="{TemplateBinding BorderThickness}">
                    <Grid RowDefinitions="Auto,Auto,Auto">
                        <ContentPresenter
                            Grid.Row="0"
                            FontWeight="Bold"
                            HorizontalAlignment="Left"
                            VerticalAlignment="Stretch"
                            Content="{TemplateBinding Header}"
                            ContentTemplate="{TemplateBinding HeaderTemplate}"
                            Margin="16" />
                        <Rectangle
                            Grid.Row="1"
                            Fill="{TemplateBinding BorderBrush}"
                            Height="1" />
                        <ContentPresenter
                            Grid.Row="2"
                            Content="{TemplateBinding Content}"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            Margin="16" />
                    </Grid>
                </Border>
            </ControlTemplate>
        </Setter>
    </ControlTheme>
</ResourceDictionary>