BabySteps

How can I do something like this?

<Style x:Key="PopupStyle" BasedOn="{StaticResource {x:Type Popup}}">
        <Style.Setters>
            <Setter Property="Template">
                <Setter.Value>
                    <Grid>

                    </Grid>
                </Setter.Value>
            </Setter>
        </Style.Setters>
</Style>