Avalonia/src/Avalonia.Themes.Fluent/Controls/FocusAdorner.xaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Styles.Resources>
    <Thickness x:Key="SystemControlFocusVisualMargin">0</Thickness>
    <Thickness x:Key="SystemControlFocusVisualPrimaryThickness">2</Thickness>
    <Thickness x:Key="SystemControlFocusVisualSecondaryThickness">1</Thickness>
  </Styles.Resources>

  <!--  HighVisibility FocusAdorner  -->
  <Style Selector=":is(Control)">
    <Setter Property="FocusAdorner">
      <FocusAdornerTemplate>
        <Border BorderThickness="{DynamicResource SystemControlFocusVisualPrimaryThickness}"
                BorderBrush="{DynamicResource SystemControlFocusVisualPrimaryBrush}"
                Margin="{DynamicResource SystemControlFocusVisualMargin}">
          <Border BorderThickness="{DynamicResource SystemControlFocusVisualSecondaryThickness}"
                  BorderBrush="{DynamicResource SystemControlFocusVisualSecondaryBrush}" />
        </Border>
      </FocusAdornerTemplate>
    </Setter>
  </Style>
</Styles>