Have you checked out the Atlas UI website? It has quite a bit of information about different aspects of styling for Atlas apps. Some helpers are documented here: https://atlas.mendix.com/p/helpers/7881299347899271
Hope that helps,
some of them are bootstrap's and can be found here: https://getbootstrap.com/docs/3.3/css/#helper-classes
But I recognize what you say. I discoverd them by just look through all the sass files and then include them in our 'living styleguide'