Some months ago this extension was added to the Miraheze catalog. While I personally don't use it, when testing it didn't work. The description on the extension page is a bit ambiguous.
The Bootstrap extension provides the Bootstrap 4 web front-end framework to MediaWiki. It can be activated directly or used from skins or extensions.
I didn't found a way to activate this directly. Is there a configuration error or something else I'm missing?
I couldn't test myself on my own host, but I did download the .zip from GitHub and apparently it only has Sass files instead of compiled CSS, is that the intended purpose?