HandyControl/src/Shared/HandyControl_Shared/Themes/Styles/Base/SeparatorBaseStyle.xaml

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

    <Style x:Key="SeparatorBaseStyle" TargetType="Separator">
        <Setter Property="Background" Value="{DynamicResource BorderBrush}"/>
    </Style>

</ResourceDictionary>