It's entirely possible to add themes to Do, but it does require coding, although it's pretty simple. It'd be technically feasible to write a theme which loaded SVGs, which could then provide easily-installable themes.

I'm not particularly interested in doing that in the immediate future - I find the existing themes sufficient, and there are problems I'm more interested in fixing - but I'd be happy to review a merge request. To do that, you'd make a branch of Do's source code, make your changes, push that branch up to launchpad, and then request a merge into trunk with bzr lp-propose lp:do.

You're welcome to hop on to #gnome-do on freenode, too. It's fairly quiet, so if you ask a question don't be surprised if it takes a while for someone to get back to you :).

Do's not dead - I've made two releases this year already, and have some core enhancements nearly ready to merge.