It isn't a short process, I can tell you that. It's all css and html, so it's not that complicated.
A couple sites I've done with Xoops:
http://www.lindavester.net http://www.ncunderground.com
I've had to modify the template parts of some of the modules themselves, and added new css code to make some parts of it.
I suggest taking the original default theme and changing it. That's what I do. All of my themes are based on the default.