This walkthrough details how to use the Themes of the Fluid Skinning System (FSS). Here, "themes" refers to the color schemes and "window dressings" for basic markup and FSS Helpers such as Tabs, Widgets, Menus, etc.
This walkthrough is meant as an accompaniment to the Theming Functional Demo, and not as an introduction to CSS or how it works. That information is out of the scope of this document and can be found at the W3C.
For an in-depth article on how the FSS uses CSS and makes it modular, please see the Fluid Skinning System (FSS) FSS - Feature Roadmap introduction.