If you're designing sites yourself, I completely agree with you. I haven't much of an eye for design myself and always rely on foundational CSS to take care of

If you're designing sites yourself, I completely agree with you. I haven't much of an eye for design myself and always rely on foundational CSS to take care of these kinds of decisions when I build pet projects; however, I've always worked on teams that have designers who hand me high-fidelity mocks (or at least high-level style guides) that answer these questions. The implementation itself is simple enough once you've worked with CSS long enough.