Avalonia/src/Avalonia.Themes.Default/GridSplitter.xaml

<Styles xmlns="https://github.com/avaloniaui">

  <Style Selector="GridSplitter">
    <Setter Property="Focusable" Value="True" />
    <Setter Property="MinWidth" Value="1" />
    <Setter Property="MinHeight" Value="1" />
    <Setter Property="Background" Value="{DynamicResource ThemeControlMidBrush}" />
    <Setter Property="PreviewContent">
      <Template>
        <Rectangle Fill="{DynamicResource HighlightBrush}" />
      </Template>
    </Setter>
    <Setter Property="Template">
      <ControlTemplate>
        <Border 
          BorderBrush="{TemplateBinding BorderBrush}"
          BorderThickness="{TemplateBinding BorderThickness}"
          CornerRadius="{TemplateBinding CornerRadius}"
          Background="{TemplateBinding Background}"/>
      </ControlTemplate>
    </Setter>
  </Style>

</Styles>