App.xaml

<Application
    x:Class="SO_75067133.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:SO_75067133"
    StartupUri="MainWindow.xaml">

    <Application.Resources>
        <Style x:Key="Style1" TargetType="Grid">
            <Setter Property="Background">
                <Setter.Value>
                    <VisualBrush Stretch="None">
                        <VisualBrush.Visual>
                            <local:MyUserControl />
                        </VisualBrush.Visual>
                    </VisualBrush>
                </Setter.Value>
            </Setter>
        </Style>

        <VisualBrush x:Key="Brush" Stretch="None">
            <VisualBrush.Visual>
                <local:MyUserControl />
            </VisualBrush.Visual>
        </VisualBrush>

        <Style x:Key="Style2" TargetType="Grid">
            <Setter Property="Background" Value="{StaticResource Brush}" />
        </Style>

        <Style x:Key="Style3" TargetType="Grid">
            <Setter Property="Background" Value="Red" />

            <Style.Triggers>
                <Trigger Property="IsMouseOver" Value="True">
                    <Setter Property="Background" Value="{StaticResource Brush}" />
                </Trigger>
            </Style.Triggers>
        </Style>
    </Application.Resources>
</Application>