MainView.axaml

<UserControl x:Class="GroupBox.Avalonia.Sample.Views.MainView"
             xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:ctrl="using:GroupBox.Avalonia.Controls"
             xmlns:themes="using:GroupBox.Avalonia.Themes">

  <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
    <Grid Margin="4"
          ColumnDefinitions="* *" RowDefinitions="* *">
      <Grid.Styles>
        <Style Selector="Grid &gt; ctrl|GroupBox">
          <Setter Property="Margin" Value="4" />
        </Style>
      </Grid.Styles>

      <ctrl:GroupBox Header="Classic"
                     Theme="{StaticResource GroupBoxClassic}"
                     Grid.Row="0" Grid.Column="0">
        <StackPanel>
          <CheckBox>CheckBox0</CheckBox>
          <CheckBox>CheckBox1</CheckBox>
        </StackPanel>
      </ctrl:GroupBox>

      <ctrl:GroupBox Header="Basic (default)"
                     Grid.Row="0" Grid.Column="1">
        <StackPanel>
          <CheckBox>CheckBox0</CheckBox>
          <CheckBox>CheckBox1</CheckBox>
        </StackPanel>
      </ctrl:GroupBox>
    </Grid>
  </ScrollViewer>
</UserControl>