I’m not comfortable with this simplistic hard rule-style advice for CSS file organization (or the comparison to glass panes), as there are big concerns against single CSS files worth mentioning: Performance is worse (multiple files is the way forward with HTTP2); organization within a single file can get messy; version control can become really obnoxious when you have to reorganize large chunks; and multiple people working on the same file becomes more prone to conflicts than when they’re separated.
If you’re only one of a very small team working on the single CSS file for a website, it’s probably fine. If you’re creating an app or working on a larger team, or both, object-oriented organization of files may well be better.
And even then, I think a lot of this is a matter of taste and personal preferences. Even for my own personal sites, I stopped liking the single file approach, despite not being that big or complicated.
And all of this isn’t even touching on the LocalCSS topic which changes *everything* about this :)