#22862: Consider a CSS preprocessor

In #22862, it was proposed that coreCore Core is the set of software required to run WordPress. The Core Development Team builds WordPress. move to using a CSSCSS Cascading Style Sheets. preprocessor. Given that we now have the appropriate tooling in place and MP6 has been greenlighted for preparation to merge into core, it is time we hash this out: which, if any, CSS preprocessor should we use, why that particular one, what does implementing it take, and what does the contribution process look like? Some of this discussion has already taken place on-ticketticket Created for both bug reports and feature development on the bug tracker., which is great, including a ton of proof-of-concept patching by @wonderboymusic.

For further reference, MP6 currently uses SCSS for its color schemes, with a branch that demonstrates what it would look like in LESS. Many thanks to @ryelle for maintaining the LESS branchbranch A directory in Subversion. WordPress uses branches to store the latest development code for each major release (3.9, 4.0, etc.). Branches are then updated with code for any minor releases of that branch. Sometimes, a major version of WordPress and its minor versions are collectively referred to as a "branch", such as "the 4.0 branch".. By no means does this dictate what happens in core itself, but it is certainly an influence and a great data point.