OverlayGeneric.xaml

<Style TargetType="{x:Type local:Overlay}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ContentControl}">
                <AdornerDecorator x:Name="PART_OverlayAdorner">
                    <ContentPresenter/>
                </AdornerDecorator>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>