How does your company manage CSS with hundreds, or even thousands of developers working with it?

I haven't seen this problem solved at any company I've worked for.

Usually there's some attempt at an internal framework or style guide, but it's always poorly documented, not maintained, and unused. Developers just write their own CSS from scratch any time it's needed which results in bloated style sheets and a nightmare to refactor.

What kind of tools and techniques do you use to make CSS smooth and efficient at your company?