Personas/Personas and Themes
Personas Plus version 1.6 adds support for simultaneous theme and persona application.
There are two methods of doing this.
The recommended method is for themes to add the attribute 'skinnable' to their install.rdf file and set it to true. This will tell Firefox personas can be applied at the same time.
Power User Method
Users can force a theme to be skinnable by adding 'lightweightThemes.forceSkinning' (boolean) to about:config and setting to true. This will force all themes to allow personas. YMMV.