Feature Request: support prefers-color-scheme

[hexylena]

I noticed there's a built in dark mode which is fantastic news! And a selector to change between them in the configuration.

Describe the solution you'd like

I think the default theme should be automatically light/dark, based on the preference sent by the user's browser.

Describe alternatives you've considered

Manually switching colors schemes works, but, it is less user-friendly than it could be.

Additional context

There's also a prefers-contrast-mode but I don't think many people use that. It's probably not necessary. And thankfully jbrowse uses few animations so prefers-reduced-motion isn't required as well <3