Board Thread:Support Requests - Getting Technical/@comment-7928705-20130828193509/@comment-1330470-20130828194426

is for JavaScript, not CSS. There'd be JS code somewhere on the wiki to make it usable.

However, it's also a really bad idea to use in the first place - the use of "collapsible collapsed" has since been superseded by. This new code is built into Mediawiki, so no additional JS code is needed.

If you change all instances of the old "collapsed", etc to the new "mw-collapsed", you should be OK. Check out Help:Collapsing.