MaterialDesignTheme.Separator.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:converters="clr-namespace:MaterialDesignThemes.Wpf.Converters"
                    xmlns:wpf="clr-namespace:MaterialDesignThemes.Wpf">
  
  <Style x:Key="MaterialDesignSeparator" TargetType="{x:Type Separator}">
    <Setter Property="Background" Value="{DynamicResource MaterialDesign.Brush.Separator.Background}" />
    <Setter Property="BorderBrush" Value="{DynamicResource MaterialDesign.Brush.Separator.Background}" />
    <Setter Property="Height" Value="1" />
    <Setter Property="MinHeight" Value="1" />
    <Setter Property="SnapsToDevicePixels" Value="true" />
    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
    <Setter Property="VerticalContentAlignment" Value="Center" />
    <Setter Property="Template">
      <Setter.Value>
        <ControlTemplate TargetType="{x:Type Separator}">
          <Rectangle Height="{TemplateBinding Height}"
                     Width="{TemplateBinding Width}"
                     HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                     VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                     Fill="{TemplateBinding Background}" />
        </ControlTemplate>
      </Setter.Value>
    </Setter>
  </Style>
</ResourceDictionary>