CSS files added to the site collection’s Style Library are effectively customized because they are stored in the SharePoint content database. SharePoint references CSS files on the Web front-end and from the SharePoint content database as needed. When you work with CSS files in SharePoint Designer, you can also access Web front-end CSS files, specifically the corev4.css file, and optionally customize that file. If you choose to customize the corev4.css file then you effectively create a local copy of that file, specific to the current site.
For further information on customizing SharePoint files, see Chapter 11.
If you open, modify, and save the corev4.css file in SharePoint Designer then a copy of the file is saved ...