The following CSS reference guide highlights the major styles used in MOSS and WSS v3 sites. The guide is organized by area of the page and include the style name, function, CSS file name and approximate line number and a sample screenshot of the style location on the screen. Use these guides to identify styles that can be overridden for your custom branding and installation or to quickly reference style name and CSS information for customization.
The default CSS files are located on the web server in the following directory: Local Drive:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\1033\STYLES.
Additional styles are located in the Publishing feature: Local Drive:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\FEATURES\PublishingLayouts\en-us
Need to Learn More About CSS?
Check out this article first: SharePoint and Cascading Style Sheets: How to update, change and reference