Respect dark mode system preference

[mtlynch]

I spoke to a customer who prefers dark mode and had to use the DarkReader extension for TinyPilot.

There's a lot we could adjust to get dark mode working, but they said that a good 80% solution would be just to fix the background, which I think is as easy as just adding this CSS rule to style.css.

 --brand-logo-accent-color: #232b32;

@media (prefers-color-scheme: dark) {
  body {
    background: var(--brand-dark-background);
  }
}