[Refactor] Switch to Emotion/theme-ui

[tesseralis]

I originally made polyhedra viewer yeeearrs ago when the hot new styling library was Aphrodite. Since then, stuff like styled components and emotion have arisen with new paradigms on how to style stuff.

Instead of switching over to that stuff, I decided it would be a wise idea to make my own weird useStyles hook, still using Aphrodite, thus getting the worst of all worlds. It's time to bring my css-in-js to the modern era (until something new comes along next year and everything becomes outdated again).

In particular, I got to use Theme UI through my work at Gatsby and it seems like it encapsulates a lot of the design choices I made (media queries, a binary scale based on rem size, etc.)

It would make it easier to do stuff like add a dark mode!