MOSS has a principal css file: core.css.

It is located in the 12 hive (of course!):

C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\LAYOUTS\1033\STYLES\CORE.CSS

If you want to customize SharePoint styles or add your own this is the place, now, don’t plan to open this file and start working on it, remember always to make a copy of the file, rename it, and then work on the copy.

 Once you are done customizing the css file, you have to tell SharePoint to use your file; to do it, open your master page in SharePoint Designer and locate the tag:

 <SharePoint:CssLink runat=”server”/>

 Use the attribute DefaultUrl to specify the location of the file

 <SharePoint:CssLink DefaultUrl=”/_layouts/1033/styles/MyCSS.css” runat=”server”/>

 Done!

 You can also specify an Alternative css file to apply to the pages when you are selecting the master page to use (using the browser).

 The order of how the styles will be applied to the pages is: 

1. Alternative CSS
2. The CSS specified in the SP control
3. CORE.CSS