Board Thread:Support Requests - Getting Technical/@comment-26840207-20150903015028/@comment-24751354-20150906000004

I've looked into it a bit. They are using some Javascript code imported from here. That's what toggles it back and forth.

It requires them to set classes to each spoiler box, through the Template:Spoiler.

They store the rest of the classes they need about halfway down their MediaWiki:Common.css list.