The portal itself comes with about a dozen or so standard themes.
I'd suggest you take a look at those ones, see which ones you like, open the
JSP files and image files related to them, and then mix and match until you get something you like. That's a safe development plan, and it will also show you how the people that know the portal the best put their themes together.
That's a good way to learn. And with a bit of experience, you can start being more and more creative.
-Cameron McKenzie