You can find all these classes in the SCSS files of your theme folder. There is not a complete list with all the classes, that I’m aware off.
You can find more information about customizing CSS in Mendix at the following pages:
I hope this helps.
Hi, IMHO the best way to check the classes in which you can use in the widgets is to open the files which are placed in the 'themesource' directory in your project.
For example, if you go to the themesource\atlas_web_content\web\buildingblocks\_heroheader.scss you can check all variants & elements for each widget & buildingblock etc.