From the get-go, I wanted Next.js to resemble "vanilla CSS" as much as possible. It took me a while to come up with the exact implementation (styled-jsx), and in the meantime I personally liked Glamor best.
So what we settled on is to embrace all CSS-in-JS systems. I think if I were to pick one from scratch today I'd use Glamor or Styled Components. Styletron has features of both and I like it as well.
At the same time, I think inline styles are also super powerful (in their simplicity!) and work amazing well for most use cases.
As a final note: CSS is actually a very nice syntax. I find that I'm really productive writing CSS! It's nice when the selectors don't leak and you can use it within component boundaries.