From b2evolution manual
This manual page is outdated. NEW PAGE: Css Files in b2evolution v5+.
If you want to make simple edits to change the look of your blog, then you don't have to edit the php files at all. You can edit the css files in a skin to change colors, fonts, sizes, background images and even the layout. One of the main advantages of css is that it separates the content from the presentation. You can learn the basics from these css tutorials. Then you can download the css file from the skin you want to edit, and make your changes in Notepad, TextEdit or some other text editor. Don't use a full-blown word processor for your edits, as it could insert formatting data that you don't need. Once you've finished your edits, save your changes and re-upload the file.
A nice way to try css changes without all that work is to use Firefox with the Web Developer Extension, or even better; the FireBug extension. You can edit css files right in the brower and see your changes immediately. When you're ready, apply those changes via ftp. This extension also gives you some tools for tracking down layout problems. You can display id and class names, outline block level elements and more.