Custom CSS class for field editor sections?

I would like to style different sections of one field editor differently (e.g. a blueish background and a green background). Unfortunately I am not able to add any CSS class to a specific section - there is no possibility to add this through the ui (maybe through XML?). Unfortunately there is also no ID that could be referenced to from the stylesheet? Does somebody have an idea to accomplish the above?

John explored this very use case in a developer deep dive that we recorded a few weeks ago.  Here is a snapshot of the end result he was able to come up with:


I think its exactly what you are after. 

Check out that dev talk at: