Earlier quoted context omitted.
The other personas (aka "lightweight themes") are supposedly being re-branded (thought I don't remember what they're calling them). It apparently hasn't happened yet, which will surely lead to some trouble.
Why don't they call them lightweight themes?
http://blog.mozilla.org/addons/2012/03/01/personas-are-joini...
The term chosen was "background theme", since a background image is essentially all that they are.
"Skin" was also under consideration.