Modular CSS needs to be developed and all inherited properties needs to commented within CSS declaration so one don’t repeat same CSS again and selectors needs to named by some naming convention (e.g. prefixing every declaration by module).

Modules needs to be devised in such way so it remain plug and play if plugged in different application.

Now, I am no expert in OOP or software design, but here’s a philosophy which might help you make the most of what you know about OOP and software design.