docs: Detect initial `prefers-color-scheme` value for `mode` localStorage

[kylejrp]

This is really a nitpick and not something too important to fix.

After having the docs site open all day, I just noticed it had a theme switcher. I'm rocking prefers-color-scheme: dark and it didn't detect it automatically. Any chance you'd consider adding a JavaScript prefers-color-scheme: dark and/or prefers-color-scheme: light and set the mode localStorage value appropriately (only if it's not already set)? 😄

https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/At-rules/@media/prefers-color-scheme

Something like:

if (localStorage.getItem('mode') === null) {
  if (window.matchMedia('(prefers-color-scheme: dark)').matches) {
    localStorage.setItem('mode', 'dark');
  } else if (window.matchMedia('(prefers-color-scheme: light)').matches) {
    localStorage.setItem('mode', 'light');
  }
}