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.
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