I had to implement a multiple colored theme for one of our sites. The site is a small company site with few static pages and a gallery. I know that there are several modules that can implement this feature, but I didn't want to install extra modules. So I added following few lines to the template.php file from my zen-based theme:
[gist:4135387] This a very tiny but powerful function. All you have to do is to create a vocabulary, and assign some terms for it. After that create some css files with the terms' name in your theme folder. Finally add this function to your template.php file, and call it with the vid of the theme-changer vocabulary. Enjoy :)
