Everything old is new again (sort of). There was a similar paradigm being promoted several years back that was dubbed "Object-Oriented CSS (OOCSS)": https://www.smashingmagazine.com/2011/12/an-introduction-to-...

It's a little unfortunate that the use of the terms "functional" and "object-oriented" were chosen in these cases as they can easily be confused with functional vs. OO programming, when in reality CSS or any paradigm of writing CSS has nothing to do with this distinction.