Rely on prefers-color-scheme w/o night mode set

[maxnordlund]

Currently when a user visits the page during the night (or rather when
prefers-color-scheme is dark), the page uses night mode.

This is great, but what's not so great is that it remembers this as a
user choice, and won't automactically change if the user visits the page
at a later time during the day (when prefers-color-scheme is light
or no-preference).

This makes it so that night mode is an explicit choice, which is still
remembered. But if the user turns off night mode it goes back to the
light colors, and if the user later visits the site during night time,
that will be picked up.

The reasoning here is that it's called night mode, which previously was
a opt-in, but the current code makes it set the preference on first
load, and never let's the user change that.

[maxnordlund]

I didn't add this to the PR but one could also use https://developer.mozilla.org/en-US/docs/Web/API/MediaQueryList to listen for changes in the users color preference.

Another way would be to switch to a CSS only solution. Basically, if the user has no-preference use the JS value, otherwise assume the user wants all of their apps to comply with the global light/dark preference.

But that's a more invasive change I think (and I don't know less to well).

[josevalim]

❤️ 💚 💙 💛 💜