2
You may want to take a look at the XOOPS For Dummies e-book published by JackJ. I'm pretty sure he has a link or two there as well as covers some of the basics of mod'ing templates.
Link:
https://xoops.org/modules/news/article.php?storyid=1519
If you have any familiarity at all with html and CSS, my suggestion would be to open up the theme.html file in an editor [text or something like Dreamweaver] and take a look at the code and start tinkering. The core code of the theme.html file is actually pretty simple. The CSS in the file style.css isn't too difficult either.
Important note: You may want to use a program like XSAS or easyphp to setup a local web server for your tinkering, lest you mess up your production site.
If you need a primer on how to get started with coding, I highly recommend
http://www.w3schools.com/.Best of luck!