CSS-in-JS has been a challenge – we're currently using https://github.com/astroturfcss/astroturf which seemed the simplest zero-runtime-cost option back when we were looking, but the library is starting to feel a bit under-maintained (if the author of Astroturf reads this, we love your work and will do whatever we can to support you in it!). But it's worked well for us over the last 2 years.

Happy to share a component example. We also use a small library for managing CSS classes in a typed fashion, which can also be used by our UI test code to target various elements. So that does add a bit of cryptic boilerplate, but the repo README has an example component with syntax highlighting: https://github.com/evnp/namespace.style#usage