Microsoft® SharePoint® Designer 2010 Unleashed

CSS Inheritance in Site Collections

Typically, when you create custom CSS files, you deploy those files to the site collection’s Style Library and reference the files in your custom master page. In SharePoint Server 2010 publishing sites, you choose to have all subsites inherit the master page, which also inherits both the custom and default CSS referenced in the master page. Alternatively, you choose to use the Alternate CSS URL setting on the SharePoint Server 2010 Site Master Page Settings page and reset all subsites to inherit the alternate CSS URL. Either way, centrally managing CSS files makes for easier administration, management, and updates.

However, you might also choose to break CSS inheritance, depending on your organization’s governance ...