Themes/MaterialDesignTheme.ToolTip.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">
  <Style x:Key="MaterialDesignToolTip" TargetType="{x:Type ToolTip}">
    <Setter Property="Background" Value="{DynamicResource MaterialDesign.Brush.ToolTip.Background}" />
    <Setter Property="CustomPopupPlacementCallback" Value="{x:Static wpf:ToolTipAssist.CustomPopupPlacementCallback}" />
    <Setter Property="Foreground" Value="{DynamicResource MaterialDesign.Brush.Background}" />
    <Setter Property="Padding" Value="8" />
    <Setter Property="Placement" Value="Custom" />
    <Setter Property="Template">
      <Setter.Value>
        <ControlTemplate TargetType="{x:Type ToolTip}">
          <Border Margin="{TemplateBinding Margin}"
                  Background="{TemplateBinding Background}"
                  CornerRadius="2">
            <ContentPresenter x:Name="ContentPresenter"
                              Margin="{TemplateBinding Padding}"
                              Content="{TemplateBinding ContentControl.Content}"
                              ContentStringFormat="{TemplateBinding ContentControl.ContentStringFormat}"
                              ContentTemplate="{TemplateBinding ContentControl.ContentTemplate}" />
          </Border>
        </ControlTemplate>
      </Setter.Value>
    </Setter>
  </Style>
</ResourceDictionary>