Board Thread:Support Requests - Getting Technical/@comment-31944763-20170509123937/@comment-168424-20170509181735

So, Fandom supports thousands and thousands of wikis. In order to do this, they don't deliver some changes (like CSS or JS) immediately after a change. They use what's called a caching system. Because of this, changes have to get checked against this system and propagate out once the system delivers highly demanded content first. Most wiki-wide CSS is not considered high demand content unless it's on a highly trafficked wiki, so most of us schlubs have to wait a few minutes for changes to show up.

Here's a good way to get your changes to show up:
 * Make sure to test your CSS at Special:MyPage/wikia.css before you put it in wiki-wide MediaWiki:Common.css or MediaWiki:Wikia.css.
 * Flush your local browser cache (Cmd/Ctrl/Shift-Reload usually does it). Best done on both the CSS page and the target page.
 * Add ?action=purge or ?&action=purge to the end of your URL on the page you're looking for the changes.
 * Wait at least 5 min.