I had needed to do this a couple months ago for a client. Rather than having class names entered into the administration of the fields as hereswhatidid did, I simply added the field name, field key, Flexible Content name (if any) and Flexible Content layout name (if any) to the body tag classes.
I decided to make a plugin to enable this functionality, though on the plugin page I also provide code that will do the same thing (FAQ tab).
https://wordpress.org/plugins/acf-wysiwyg-styling/
