Vector and Minerva now have identical skin variables definitions that need to be manually synced.
They live here:
* https://github.com/wikimedia/mediawiki-skins-MinervaNeue/blob/master/resources/mediawiki.less/mediawiki.skin.variables.less
* https://github.com/wikimedia/Vector/blob/master/resources/mediawiki.less/mediawiki.skin.variables.less
Going forward, we would like to define a Wikimedia theme that both Minerva and Vector can consume.
One solution would be to put this shared variables list in core, but this has the negative effect of making all MediaWiki instances look like Wikimedia's brand.
I wrote some notes here https://www.mediawiki.org/wiki/User:Jdlrobson/Themes_in_MediaWiki
One idea is too make it possible to set themes on skins which would allow us to define themes for dark mode for example which is a longstanding request (T26070).