As reported at https://www.mediawiki.org/wiki/Thread:Extension_talk:MobileFrontend/Common.css_ignored%3F, the MobileFrontend extension apparently load MediaWiki:Common.css. MediaWiki:Common.css is the styling that should apply to the entire site.
Looking at the page source of https://en.m.wikipedia.org/ seems to confirm that MediaWiki:Common.css is currently completely ignored.
Version: unspecified
Severity: normal