Avalonia/samples/ControlCatalog/Pages/BorderPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.BorderPage">
  <StackPanel Orientation="Vertical" Spacing="4">
    <TextBlock Classes="h1">Border</TextBlock>
    <TextBlock Classes="h2">A control which decorates a child with a border and background</TextBlock>

    <StackPanel Orientation="Vertical"
                Margin="0,16,0,0"
                HorizontalAlignment="Center"
                Spacing="16">
      <Border BorderBrush="{DynamicResource ThemeAccentBrush}" BorderThickness="2" Padding="16">
        <TextBlock>Border</TextBlock>
      </Border>
      <Border Background="{DynamicResource ThemeAccentBrush2}" 
              BorderBrush="{DynamicResource ThemeAccentBrush}" 
              BorderThickness="4" 
              Padding="16">
        <TextBlock>Border and Background</TextBlock>
      </Border>
      <Border BorderBrush="{DynamicResource ThemeAccentBrush}"
              BorderThickness="4" 
              CornerRadius="8"
              Padding="16">
        <TextBlock>Rounded Corners</TextBlock>
      </Border>
      <Border Background="{DynamicResource ThemeAccentBrush2}"
              CornerRadius="8"
              Padding="16">
        <TextBlock>Rounded Corners</TextBlock>
      </Border>
    </StackPanel>    
  </StackPanel>
</UserControl>