If you are strictly a CSS guy, check out Thematic. It starts out as a theme that has all the basics (header, navigation, content, sidebar, footer) without any images. You can start a child theme from this with a blank CSS file and work from there. If you want to edit the PHP though, good luck.

Themeforest.com

If you want to work with a theme that already "exists", check out Thesis. It provides the framework and hooks that let you build your own theme in a very customizable and flexible way.