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.
Would Twitter be the same if it was for short text messages? Or are Tweets so different that they deserve their own name?
The term chosen was "background theme", since a background image is essentially all that they are.
"Skin" was also under consideration.