Avalonia/samples/ControlCatalog/Pages/ContentPage/ContentPageCustomizationPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.ContentPageCustomizationPage">
  <DockPanel>
    <ScrollViewer DockPanel.Dock="Right" Width="260">
      <StackPanel Margin="12" Spacing="8">
        <TextBlock Text="Configuration" FontWeight="SemiBold" FontSize="16"
                   Foreground="{DynamicResource SystemControlHighlightAccentBrush}" />

        <TextBlock Text="Background" FontWeight="SemiBold" />
        <ComboBox x:Name="BackgroundCombo"
                  SelectedIndex="0"
                  HorizontalAlignment="Stretch"
                  SelectionChanged="OnBackgroundChanged">
          <ComboBoxItem Content="Default" />
          <ComboBoxItem Content="Light Blue" />
          <ComboBoxItem Content="Light Green" />
          <ComboBoxItem Content="Light Purple" />
          <ComboBoxItem Content="Warm White" />
        </ComboBox>

        <Separator />

        <TextBlock Text="Horizontal Content Alignment" FontWeight="SemiBold" />
        <ComboBox x:Name="HAlignCombo"
                  SelectedIndex="3"
                  HorizontalAlignment="Stretch"
                  SelectionChanged="OnHAlignChanged">
          <ComboBoxItem Content="Left" />
          <ComboBoxItem Content="Center" />
          <ComboBoxItem Content="Right" />
          <ComboBoxItem Content="Stretch" />
        </ComboBox>

        <TextBlock Text="Vertical Content Alignment" FontWeight="SemiBold" />
        <ComboBox x:Name="VAlignCombo"
                  SelectedIndex="3"
                  HorizontalAlignment="Stretch"
                  SelectionChanged="OnVAlignChanged">
          <ComboBoxItem Content="Top" />
          <ComboBoxItem Content="Center" />
          <ComboBoxItem Content="Bottom" />
          <ComboBoxItem Content="Stretch" />
        </ComboBox>

        <Separator />

        <TextBlock Text="Padding" FontWeight="SemiBold" />
        <Slider x:Name="PaddingSlider"
                Minimum="0"
                Maximum="48"
                Value="0"
                TickFrequency="8"
                IsSnapToTickEnabled="True"
                ValueChanged="OnPaddingChanged" />
        <TextBlock x:Name="PaddingLabel"
                   Text="0 px"
                   HorizontalAlignment="Center"
                   Opacity="0.7" />

        <Separator />

        <TextBlock Text="About" FontWeight="SemiBold" />
        <TextBlock Text="• HorizontalContentAlignment / VerticalContentAlignment position the content inside the page area."
                   FontSize="12" Opacity="0.7" TextWrapping="Wrap" />
        <TextBlock Text="• Padding adds space between the page border and its content presenter."
                   FontSize="12" Opacity="0.7" TextWrapping="Wrap" />
        <TextBlock Text="• Background accepts any Avalonia brush, including gradients."
                   FontSize="12" Opacity="0.7" TextWrapping="Wrap" />
      </StackPanel>
    </ScrollViewer>

    <Border DockPanel.Dock="Right" Width="1" Background="{DynamicResource SystemControlForegroundBaseMediumLowBrush}" />

    <Border Margin="12"
            BorderBrush="{DynamicResource SystemControlForegroundBaseMediumLowBrush}"
            BorderThickness="1"
            CornerRadius="6"
            ClipToBounds="True">
      <ContentPage x:Name="SamplePage"
                   Header="Customization"
                   HorizontalContentAlignment="Stretch"
                   VerticalContentAlignment="Stretch">
        <Border Background="#E0E0E0"
                CornerRadius="8"
                Padding="24"
                Width="180"
                Height="100">
          <StackPanel Spacing="4"
                      HorizontalAlignment="Center"
                      VerticalAlignment="Center">
            <TextBlock Text="Content Area"
                       FontSize="16"
                       FontWeight="SemiBold"
                       HorizontalAlignment="Center" />
            <TextBlock Text="Adjust alignment and padding"
                       FontSize="12"
                       Opacity="0.6"
                       TextWrapping="Wrap"
                       TextAlignment="Center" />
          </StackPanel>
        </Border>
      </ContentPage>
    </Border>
  </DockPanel>
</UserControl>