Many users have limited html or css skills and find it difficult to locate which CSS file to edit.
Just about every skilled and knowledgeable webmaster, developer or designer uses Google Chrome's "Element Inspector" feature. It makes it quick and easy to locate which CSS files and code are associated with an element within a certain page.
Here's some YouTube tutorials on the basics of using Google Chrome's Element Inspector.
This video is a basic introductory on understanding how it works:https://www.youtube.com/watch?v=DPqaGWllYYQ
This video explains in more detail:https://www.youtube.com/watch?v=nV9PLPFTnkE