Avalonia/src/Avalonia.Themes.Simple/Controls/AdornerLayer.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">
  <ControlTheme x:Key="{x:Type AdornerLayer}" TargetType="AdornerLayer">
    <Setter Property="DefaultFocusAdorner">
      <FocusAdornerTemplate>
        <Rectangle Margin="1"
                   Stroke="Black"
                   StrokeDashArray="1,2"
                   StrokeThickness="1" />
      </FocusAdornerTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>