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

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">
  <ControlTheme x:Key="{x:Type GridSplitter}"
                TargetType="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 Background="{TemplateBinding Background}"
                BorderBrush="{TemplateBinding BorderBrush}"
                BorderThickness="{TemplateBinding BorderThickness}"
                CornerRadius="{TemplateBinding CornerRadius}" />
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>