(Edit: See #5533 for a fresh attempt at this.)

Over the last year or two it's become common for apps to have both a dark and a light color scheme, and let the user choose between them. Zulip has this.

Since 2019 this has become a thing at the OS level. And in recent months, I've seen more and more apps noticing my OS-level setting and following along. We should do the same, at least by default -- when the user goes into system-wide settings and says they want the device's screen to stay dark instead of bright, we shouldn't wait for an individual invitation.

This becomes especially significant now that it's an OS-level feature, in both Android and iOS, to have the light/dark mode switch back and forth every morning and evening. The only way that can work as a good experience is when apps all follow along automatically.

RN v0.62 adds an API for this. So I think it makes sense to hold off on trying to implement this behavior until we've done that upgrade, aka #3782. Done!