Did you check out the SASS files? It should be next to the CSS files. In the LIB map you can look them up
I hope that is what you need.
Also take a look at http://atlas.mendix.com . It's all about the new Atlas Framework.
On the site, there's also a specific list for helper classes: https://atlas.mendix.com/p/helpers/7881299347899271
Quote: In addition, these classes seem to be unique to Mendix, so the Bootstrap CSS page does not have them.
Mendix has itnegrated bootstrap 3 to some extent so some classes from Bootstrap are available to use. Forr example the glyphicons, column layouts e.g. col-md-4 also btn-lg, clearfix, text-center, content-center to name a few.
A comprehensive list can be found here: http://bootstrapdocs.com/v3.0.3/docs/css/#buttons