Respect user preference for color scheme

[abhishalya]

Automatically switches to 'non-primary' theme if user prefers dark scheme.
If there is a theme specified in the local storage then that takes more preference over this.

Closes #1320

[mortenpi]

Currently it looks like it always defaults to the dark theme now, even though

> window.matchMedia('(prefers-color-scheme: dark)')
MediaQueryList {media: "(prefers-color-scheme: dark)", matches: false, onchange: null}

So it feels like something is broken here.

[abhishalya]

Tried to address all above comments, also confirmed now that it works as it is supposed to.

[mortenpi]

Awesome, seems to work nicely now! A few more things that popped up as I was playing around with this:

1. There is the if(typeof(window.localStorage) === "undefined") return; at the beginning of set_theme_from_local_storage().

   This will mean that in browsers where window.localStorage is undefined, but that prefer the dark theme, the theme will not be changed. So I think this should now be removed and the code below updated to make sure we handle the case where there is no localStorage.

   I also discovered that window.localStorage can be null (e.g. if you disable localStorage). So I also think we should check with window.localStorage == null instead.

2. The theme picker in the settings dialog currently still shows "documenter-light" if the dark theme is selected by the browser. So we should update that code as well.

[mortenpi]

It looks like I can push to this branch myself. Could you also bring the branch up to date with master and cherry-pick this CHANELOG commit: 2751b43.

Other than this and the one code suggestion, I think this is good to go 🙂

[mortenpi]

Could you merge master into this again? Or tick the "allow maintainers to push to this branch"? I can't merge this at the moment, because CHANGELOG.md conflicts.

[abhishalya]

Or tick the "allow maintainers to push to this branch"?

Sorry, since this is created from the MLH fork, I wasn't able to do that. Although I've asked Cory to give you access to the MLH repos, so you should be able to push directly :)

I've rebased the branch with master for now.

[mortenpi]

Thanks @abhishalya!