ResizeHandle.xaml

<Thumb x:Class="Rule.ResizeHandle"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="70" d:DesignWidth="5"
       DragDelta="Thumb_DragDelta" DragStarted="Thumb_DragStarted" DragCompleted="Thumb_DragCompleted">
    
    <Thumb.Template>
        <ControlTemplate TargetType="Thumb">
            <Border x:Name="highlight" Opacity="0" Background="{DynamicResource woodHighlightBrush}"/>

            <ControlTemplate.Triggers>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter TargetName="highlight" Property="Opacity" Value="1"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </Thumb.Template>
</Thumb>