Themes/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 MaterialDesign.Brush.GridSplitter.PreviewBackground}" />
        </ControlTemplate>
      </Setter.Value>
    </Setter>
  </Style>
  <Style x:Key="MaterialDesignGridSplitter" TargetType="{x:Type GridSplitter}">
    <Setter Property="Background" Value="{DynamicResource MaterialDesign.Brush.GridSplitter.Background}" />
    <Setter Property="Opacity" Value=".56" />
    <Setter Property="PreviewStyle" Value="{StaticResource MaterialDesignGridSplitterPreview}" />
    <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 Background="{TemplateBinding Background}"
                  BorderBrush="{TemplateBinding BorderBrush}"
                  BorderThickness="{TemplateBinding BorderThickness}" />
          <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>