There is still no customization support. For example how to change the "certain tone of blue associated with Facebook" as mentioned in MediaWiki UI#I don't like the design!? The Living style guide linked therein does not describe it, logically because the software does not allow it (see bug T100896). Considering that virtually every custom skin using custom colors has been broken, I'm wondering what was really the point of these changes. The antispam filter does not let me create a link to your bug tracker. Come on!
Topic on Talk:MediaWiki UI
From my understanding there are three different issues that are better fitted in Phabricator than here. Therefore closing this.