Running 6.2.3 on a farm using the Content Resource Management.
When site is built from template, I log in to the site, check the override host settings checkbox and click update.
I make whatever changes I need to the portal.css, and save it.
I go back into Site Management and uncheck the override host settings checkbox and click update.
Most times when I do this, everything works. Occasionally, when the box is unchecked and updated, the site reverts back to the previous css.
If I clear the cache, it will display properly until I log out, then it reverts to the old css. The only surefire fix I have found is to delete the actual .cdC file out of App_Data/ClientDependency.