People often ask where to find css resources such as overview of classes.
we have:
UX.Mendix
https://ux.mendix.com/theme-viewer.html
no idea if this is up to date but judging from the github link last update was Feb 2016. ( https://github.com/mendix/mendix-ui-framework-lib )
Atlas UI
altas.mendix.com
seems like a marketing website more then a documentation of css classes. Atlas does get reguler updates (last update Feb 2019) ( https://github.com/mendix/Atlas-UI-Framework )
Bootstrap3
bootstrap classes all seem to work in mendix, not sure which version mendix uses, definitly a version of bootstrap3 ( https://getbootstrap.com/docs/3.3/ )
Community
The UX community have a lot of solutions, bug fixes and best practises that are not aligned, and are floating around everywhere.
Solution:
Clear communication goes a long way.
Having best practises created with the community would also help a lot.
And I almost think docs.mendix should have a css overview (but might get out of hand with all the css combinations you can make, especially with how mx8 handles helper classes.
A clear overview of what's possible would go a long way and personally I agree there should be some documentation about this. Thanks for summing up some links here Jason!