MaterialDesignTheme.RichTextBox.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:wpf="clr-namespace:MaterialDesignThemes.Wpf">
    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="pack://application:,,,/MaterialDesignThemes.Wpf;component/Themes/MaterialDesignTheme.TextBox.xaml" />
    </ResourceDictionary.MergedDictionaries>

    <Style x:Key="MaterialDesignRichTextBox" TargetType="RichTextBox" BasedOn="{StaticResource MaterialDesignTextBoxBase}">
        <Setter Property="wpf:TextFieldAssist.TextBoxViewMargin" Value="-4 0 1 0" />
    </Style>
</ResourceDictionary>