MaterialDesignTheme.GridSplitter.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Style x:Key="MaterialDesignGridSplitterPreview">
        <Setter Property="Control.Template">
            <Setter.Value>
                <ControlTemplate>
                    <Rectangle Fill="{DynamicResource MaterialDesignSelection}"/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <Style x:Key="MaterialDesignGridSplitter" TargetType="{x:Type GridSplitter}">
        <Setter Property="Background" Value="{DynamicResource MaterialDesignDivider}"/>
        <Setter Property="PreviewStyle" Value="{StaticResource MaterialDesignGridSplitterPreview}"/>
        <Setter Property="Opacity" Value=".56"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type GridSplitter}">
                    <ControlTemplate.Resources>
                        <Storyboard x:Key="MouseEnter">
                            <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Opacity)">
                                <EasingDoubleKeyFrame KeyTime="0" Value="0.56"/>
                                <EasingDoubleKeyFrame KeyTime="0:0:0.2" Value="1"/>
                            </DoubleAnimationUsingKeyFrames>
                        </Storyboard>
                        <Storyboard x:Key="MouseExit">
                            <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Opacity)">
                                <EasingDoubleKeyFrame KeyTime="0" Value="0.56"/>
                                <EasingDoubleKeyFrame KeyTime="0:0:0.2" Value="0.56"/>
                            </DoubleAnimationUsingKeyFrames>
                        </Storyboard>
                    </ControlTemplate.Resources>
                    <Border BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}"/>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsMouseOver" Value="True">
                            <Trigger.ExitActions>
                                <BeginStoryboard x:Name="MouseExit_BeginStoryboard" Storyboard="{StaticResource MouseExit}"/>
                            </Trigger.ExitActions>
                            <Trigger.EnterActions>
                                <BeginStoryboard x:Name="MouseEnter_BeginStoryboard" Storyboard="{StaticResource MouseEnter}"/>
                            </Trigger.EnterActions>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>