As a front-end developer, I have my own standard in how to organise CSS files. I prefer to divide my CSS code into different modules instead of a huge file with all the styles in it. The initial setup might take a little bit of times, but in long term perspective, it's actually increase the maintainability and readability.
If you want to learn how to do it, here we have a good read called MaintainableCSS. It consists of 8 chapters - Introduction, Semantics, Reuse, IDs, Conventions, Modules, State and Versioning. In each chapter, you will learn the best way to write maintainable CSS with explanation and examples provided. You will able to learn why we use class name instead of ID, what are module and component and etc.
This online e-book are available for free, be sure to bookmark and learn all the goodies from the author.