From b2evolution manual
This manual page is outdated. NEW PAGE: Edit Evoskins in b2evolution v5+.
 CSS files
Being able to change between pre-made evoskins is great, but sometimes you want to customize your skin a little further. If you just want to change colors, column widths or background images, then all you need to do is edit the css file for your skin. Look in /skins/[name of your skin here]/ and find the file or files that end in .css .
For more, see Css files.
 _main.php file
But if you want to make more advanced changes, you'll need to edit the actual php files that make up the skin. The most important of these is _main.php. By editing the _main.php file, you can change almost anything about the way your blog displays.
For more, see _main.php file.
Note: in b2evo 2.0 and above, _main.php had been renamed to main.tpl.php; see Skins 2.0.
See Also: Create a new skin