Tweaks for dark mode

[nwalters512]

This started out as simply a fix to the error page styles (which had some hardcoded colors) but ended up containing a bunch of little refinements:

• Add a page of documentation to dark mode (how it works + emphasis on making sure new features play nicely with it)
• Persist state in a cookie so that we can SSR with/without dark mode styles as appropriate
• Adjust placement of dark mode toggle in the navbar
• Make login page dark mode compatible
• Adjust naming of properties in the theme context
• Changes the "settings" button on queue cards to a gear icon (looks better in general, but especially in dark mode)

[vercel[bot]]

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