Default font was decided to be Montserrat. We should implement this in Commons.css rather than needing div headers on each page. As a benefit it should work with tables etc.
Just as a note, a couple years ago I developed a MediaWiki extension that serves Montserrat as a web font. https://www.mediawiki.org/wiki/Extension:MontserratFont
It even passed security review!
Note that it, by itself, does not change the font on the wiki. It just makes the web font available via ResourceLoader. You must use it in conjunction with CSS rules.