This is a great new addition to Firefox 4 Themes Rated 5 out of 5 stars

Applying Personas Themes on top of custom themes?

OK, I stumbled upon this solution for Applying a Persona(lightweight) Theme to any Third Party Theme without using any extension. View my post here: https://forums.mozilla.org/addons/viewtopic.php?f=30&t=2636

This review is for a previous version of the add-on (2.5.0).