Because the desktop version of Vector looks really messed up when I load it the first time (reload fixes it, screenshot: https://abload.de/img/screenshot_2019-01-1690kds.png), I installed Extension:MobileFrontend and Skin:Minerva Neue. Now it loads the Minerva skin on phones, and that doesn't look messed up.
Now I am trying to adapt our templates such as infoboxes for the new mobile skin, but the problem is, that MediaWiki:Mobile.css doesn't load at all in the mobile version, not even as overwritten by other CSS rules, as you can see here: https://abload.de/img/mobilecssbug2pk9h.png (trying to fix the messed-up infobox). What is the problem here? Is it a bug in either Minerva or MobileFrontend?