Looks like Gmail is rolling out themes functionality over the next few days.
I’ll be interested to have a look once the roll out reached my account. Up until now I’ve been pretty happy with the Gmail Redesigned Firefox plugin for Gmail and GCal, but it would be nice to have something that isn’t browser or computer dependent.
EDITED TO ADD:
I’ve got them now, and they look pretty good.
(And if you know what a grep is, I highly recommend the terminal theme 🙂