They've resolved it in planning (http://www.ghacks.net/2012/03/02/mozilla-personas-renamed-to...), but the implementation just hasn't been completed. Yet.

"To sum it up:

* BrowserID renamed to Mozilla Persona

* Personas renamed to Background Themes

* Themes are now Complete Themes"

Background Themes/Complete Themes - sounds like they just created a new problem. Throwing the word 'theme' in there for backgrounds is just asking for confusion.

I think most people understand Wallpapers (Backgrounds) / Themes.