You can use this plugin to add a unique CSS class to your pages or posts.
Demo
TasteWP has prepared a quick WordPress instance with this plugin, so you can give it a try
in this demo
Responsiveness
There's a way to add a class to appear only on mobile devices. You can do that by simply adding
mobile-
on the front of
the class. For example, if you want to have the class "car" for mobile side, you need to use "mobile-car".
Support me
If you find this plugin helpful, or you just want to send me a coffee here are the ways: