But this still isn't a complete solution -- if you want the same color as a "color" in one place, "background-color" in another, and "border-top-color" in another, you're still repeating yourself. Likewise if you want to do math with paddings/margins etc.

I don't know why, if you're using LESS or equivalent, you wouldn't use variables. It makes for vastly more maintainable CSS than the approach you take.