JBoss.orgCommunity Documentation
By using WCM, all the stylesheets of each site can be managed online easily. You do not need to access the file system to modify and wait until the server has been restarted. For the structure, each site has its own CSS folder which can contain one or more CSS files. These CSS files have the data, and the priority. If they have the same CSS definition, the higher priority will be applied. You can also disable some of them to make sure the disabled style will no longer be applied into the site.
For example, a WCM demo package has two sites by default: ACME and Classic. The Classic site has a CSS folder which contains a CSS file called DefaultStylesheet. Most of the stylesheets of this site are defined within this stylesheet. Moreover, the ACME site has two CSS files called BlueStylesheet and GreenStylesheet. The blue one is enabled and the green one is disabled by default. All you need to test is to disable the blue one (by editing it and setting Available to 'false') and enable the green one. Now, back to the homepage and see the magic.
Remember the cache and refresh the browser first if you do not see any changes. Normally, this is the main reason why the new style is not applied.