Max Semenick prepared a patchset that would make it possible to keep Mediawiki:Common.css and friend in sync between enwiki in production, and enwiki on betalabs, but it needs review and merge: https://gerrit.wikimedia.org/r/#/c/68309/

One of the big challenges in deploying MobileFrontend changes to production is the unexpected ways Mediawiki:Common.css and friends will affect our changes. In general, the mobile web team has been trying to get betalabs to a state that mimics production closely enough that we can reliably test our changes there for all the weird things that can happen in the production environment - before pushing our changes to production. As such, it would be enormously helpful if we had a way to keep Mediawiki:Common.css and friends in sync between enwiki in production and enwiki on betalabs.

So, can someone please take a look? It would be enormously helpful for us to get this in place as soon as possible.