While WP Popups makes it really easy to edit the appearance on some occasions you need to go a bit further and add your own magic. To add custom CSS all you need to do is go to Appearance -> Custom Css and add your own code.
In order to affect only the popup you are working with, and not all of them, you need to start the rules with the popup ID prefix #spu-XXX (replace XXX for your popup ID).
Tip: You can view the ID of the popup by hovering the help tooltip of the Custom CSS box.
