HandyControl/src/Shared/HandyControl_Shared/Themes/Styles/Watermark.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:hc="clr-namespace:HandyControl.Controls">

    <Style BasedOn="{StaticResource WatermarkBaseStyle}" TargetType="hc:Watermark"/>

</ResourceDictionary>