No, not custom at all. Quite the opposite - it's a well-curated set of standard classes. It's the focus on utility-based composition of style that sets Tailwind apart. For me the problem being solved is maintainability, recognising that CSS is tremendously brittle with regard to the application it serves and the context in which it is evaluated.

Probably best to read https://adamwathan.me/css-utility-classes-and-separation-of-... which does a much better job of explaining the motivations, and the conceptual journey of its creator, than I ever could.

Many folks (not all, but many, I'd even venture to say ... most) have a four step reaction to Tailwind:

1. That is the stupidest webdev idea I ever heard, and I've heard a lot. // 2. Fine, I'm trying it because everyone else keeps prompting me to and/or its creator seems competent. // 3. This is weird but actually quite helpful and totally not what I expected. // 4. You have ruined me for any other CSS framework and I never want to use anything else.

All CSS is crap. Tailwind is (in my experience and opinion) the least crap and most productive approach yet.