As per my research, most (if not all) companies separate them and either use a build-step or a dynamic content generator to mix and optimize them ;)
Here some advice which is always important: When writing code, write it in a human-friendly way. Make sure that it is easy to read and understand. Separate concerns. Keep it simple & stupid (KISS). Try not to repeat yourself (DRY), so you have a single source of truth.
When you are done, use a build step or a compiler or a transpiler to optimize the code for machine-interpretation (that includes stuff, like minification, combining files, removing dead code and comments, etc.)
So, to answer your question: Put them into separate files. CSS in a separate file was a remarkable advancement back in the days, so why give that up?
Software Engineer, Technical Consultant & Mentor