Shrlalgo.RvKits/WPFluent/Controls/GridView/GridViewHeaderRowIndicator.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <ControlTemplate TargetType="Separator" x:Key="GridViewHeaderRowIndicatorTemplate">
        <Border>
            <Rectangle
                Fill="{DynamicResource SystemFillColorAttentionBrush}"
                Height="18"
                HorizontalAlignment="Left"
                Margin="0"
                RadiusX="2"
                RadiusY="2"
                VerticalAlignment="Center"
                Width="3" />
        </Border>
    </ControlTemplate>
</ResourceDictionary>