Board Thread:Support Requests - Designing Your Wiki/@comment-27179086-20200518052630/@comment-9605025-20200518073709

When there is a space between words in the class attribute, they are separate classes, not a single class with a space in it. So both templates belong to the "box" class and the first one also belongs to the "module" class.

The CSS they use for this is actually neither in nor imported by any of the main CSS pages. Rather, for some reason, they import the CSS via JS. Either way, I believe the relevant CSS is on MediaWiki:Frames1pe.css.