I've done some somewhat similar efforts in the past. One thing to point out is that on the something.original.css and something.refreshed.css to avoid code duplication: since y'all were already using LESS as a build process, code duplication shouldn't be a huge problem with LESS importing the original from the refreshed. I found the LESS compiler to be quite good at coalescing complicated overrides of imported base styles to a final, simpler final build output.

Though the last time I did this sort of thing I definitely couldn't have used CSS Variables. That seems like a good solution for modern browsers with or without a compiler like LESS.