CSS classes do not translate well to on-screen elements. When I want to see the CSS that affects an element in a web page, I use the browser's "Inspect Element" tool to display all the CSS related to that element. Gnome3 lacks that capability. More to the point, I come to Gnome3 as a user. I do not want to be forced to resort to manual coding, even if I have the skills. That's a mark of Gnome's shortcoming.