I haven't written anything up but feel free to hit me up with any questions as you go.

I did a very SMACSS-y folder organization. All modules in a src/modules directory & subdirs, src/base, directory, src/layouts, /src/mixins, etc.

In the src directory I have multiple top-level stylesheets (four for each brand, as I've chunked up the styles). I basically have my preprocessor crunch each of these and output each of them compiled in a build directory.

The CSS lives completely in its own repository. Published with semver & release notes. I kinda sorta wrote about my thoughts regarding that here: https://keithjgrant.com/posts/2015/08/css-first/