Usually we don’t take code organisation and architectural discipline seriously when writing stylesheets, because it’s “just CSS”. But over time, when your product grows, things get ugly. Changes in one component break the other, implementing a facelift on a shared component makes the whole product fall apart.
CSS is code, and just like any other code, it needs to have a design, be refactored time to time and adhere to set of common principles in order to scale. As any other language, it needs to be learned and it is hard. Don’t be like this guy.