aistudio-wpf-diagram/AIStudio.Wpf.DiagramDesigner/Styles/ToolTip.xaml

<ResourceDictionary 
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Shared.xaml" />
    </ResourceDictionary.MergedDictionaries>

    <!-- SimpleStyles: ToolTip -->
    <Style x:Key="{x:Type ToolTip}" TargetType="ToolTip">
        <Setter Property="OverridesDefaultStyle" Value="true"/>
        <Setter Property="HasDropShadow" Value="True"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="ToolTip">
                    <Border Name="Border"
            Background="{StaticResource LightBrush}"
            BorderBrush="{StaticResource SolidBorderBrush}"
            BorderThickness="1"
            Width="{TemplateBinding Width}"
            Height="{TemplateBinding Height}">
                        <ContentPresenter
              Margin="4" 
              HorizontalAlignment="Left"
              VerticalAlignment="Top" />
                    </Border>
                    <ControlTemplate.Triggers>
                        <Trigger Property="HasDropShadow" Value="true">
                            <Setter TargetName="Border" Property="CornerRadius" Value="4"/>
                            <Setter TargetName="Border" Property="SnapsToDevicePixels" Value="true"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>