OnlineMonitorSystem/HandyControl/HandyControl_Shared/Themes/Styles/Base/WatermarkBaseStyle.xaml

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

    <Style x:Key="WatermarkBaseStyle" TargetType="hc:Watermark">
        <Setter Property="MarkBrush" Value="{DynamicResource DarkMaskBrush}"/>
        <Setter Property="Foreground" Value="{DynamicResource DarkMaskBrush}"/>
        <Setter Property="Angle" Value="20"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="hc:Watermark">
                    <Border Name="PART_Root">
                        <ContentPresenter/>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>