I'll plug my Sass framework Eyeglass here. It allows you to distribute Sass files and related assets as npm modules. At my work we use this to distribute re-usable parts of our style like our theming system to many applications.
Lots of people like to have a variables file and mixins and partials directories. I'm really not a fan of this approach. Keep related code together and minimize bouncing around from file to file.
To that end, within an application, I suggest organization by product and feature with cross cutting theme or design language shared by all of those.