Dark mode support

[thatbudakguy]

• Add dark mode theme switcher
  This adds a new component to the main navbar for switching between
  themes, based on Bootstrap's docs and provided example for how
  to implement a dark mode toggle.

  Also adds related JavaScript for detecting and saving the user's
  preference in localStorage, and updating the switcher to indicate
  which theme is active.

• Update styles for dark mode compatibility
  This tweaks the existing styles to handle cases where colors
  were not set up to handle dark mode.

  Also replaces the logo image with two SVG versions; one light
  and one dark.

[jcoyne]

can we just do this:

@media (prefers-color-scheme: dark) {
    :root {
        --bs-body-color: white;
        --bs-body-bg: darkgray;
        /* etc. etc. */
    }
  }
}

And not have a switcher?

[thatbudakguy]

That's what the "auto" mode in the switcher does – we could also make that the default instead of light?

In general I think the approach of just detecting the user's browser/OS preference is fine, but I've heard that it doesn't work for everyone – some people want to be able to control that preference on a per-app basis, because the main content of the app affects how they read (is it a long page of text? are there lots of fiddly UI elements? etc).

Enabling it without a switcher could also lead to confusion if someone gets the dark theme but doesn't know why and can't opt back into the light theme.