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Very simply put, XOOPS works with
- content
- presentation
The content is done by HTML and PHP files, while the presentation (at least to a large extend) is done by CSS (cascading style sheets).
So a good place to start would be to look into CSS, the syntax and how you make it work with your site (be it a XOOPS site or otherwise).
Possibly it is possible to find some sort of introduction/manual for XOOPS-related themes here at XOOPS.org.