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Most (if not all?) themes have a style.css file. Some have styleNN.css and styleIE.css as well.
The style.css file is where most of your theme's css is located. The site logo, left/center/right columns, header, footer, and many more general css settings are here. Many of the standard modules use css settings from this file. The most common are odd, even, blockTitle, blockContent, etc.
The styleNN.css and styleIE.css files are for css changes that specifically affect Netscape and Internet Explorer respectively. These are exceptions. There are some known rendering issues with IE for example and some workarounds to make it render the same as NN (and vice versa). These files allow you to make those fine tuning changes.
Some modules also include css files. Their filenames can vary. This is especially true of modules that are ported from outside of XOOPS or that were developed as stand-alone programs before being brought into Xoops. Integrating this css into XOOPS can sometimes be challenging if the module redefines classes.