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Themes are extremely easy to code. The hardest part is making sure you have covered all the bases in your css stylesheet. You can do just about anything you want with a XOOPS theme. Your only limitation is your imagination.
You don't need to know php to code themes for Xoops, but here's a good reference if you need one.
http://www.w3schools.com/
The only thing you need to keep in mind about the php is it will be generating dyanamic html. In other words, don't make your page length fixed.