These are some of the style guides I have researched. These may enable a small company to have a cost effective style guide by borrowing from something already created. By forking one of these repositories and adapting it something, something, blaa, blaa nobody reads this anyway. Just look at the links below.
MIT license
I can't find specific license details or a corresponding github repo but Mapbox says:
"Feel free to find inspiration or borrow from Base. Just don't link to our CSS files directly. They could change at any minute and include some styles specific to our brand."
MIT license
Apache License 2.0
MIT license