Default themable CSS files are located on the Web front-end server in the Themable directory on the Web front-end server at %SystemDrive%\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\LCID\STYLES\Themable\.
You should avoid opening and modifying those CSS files stored on the Web front-end server. Modifying those CSS files customizes them and possibly breaks themable properties. Any modifications made are overridden by any subsequent SharePoint Service Packs.
The following default CSS files are located in the Themable directory on the Web front-end server:
If you add your own custom themable CSS files to this same location, then those files are themed ...