I've you understand dutch you may use my tutorial otherwise you can translate it.
Click here and go to the document on Google Docs.
(please click next in the right corner of your screen to continue to the document!)
What does it contains?
It contains a little tutorial and it contains a list of all the CSS classes used in mendix and on which row they are when you open the CSS document for the first time.
The DX release should have more styling information that is already prepared in SASS format to make life easier