Respect "prefers-color-scheme" media selector for guests

[nickvergessen]

Guests now automatically see dark theme when the browser tells the website it prefers a dark style:
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme

A similar setting for contrast high seems to be in development but is not supported:
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-contrast

See https://stackoverflow.com/a/56757527 for helps how to test this quickly

Edit by @georgehrke:
fixes #12276

[nickvergessen]

cc @nextcloud/accessibility

[nickvergessen]

cc @nextcloud/theming

[jancborchardt]

Wow, this is pretty amazing! :) Great integration and works well when tested with the link sharing pages of Forms, Talk, and Files.

[georgehrke]

Where does the actual @media (prefers-color-scheme: dark) { come from though?

[nickvergessen]

Its on the link meta tag, that is the only why you can set global css vars within a condition

[juliusknorr]

Does not fully fix #12276 as it is only for unauthorized access though public pages, but a good first step 👍

[nickvergessen]

Yeah, we can show a not about the personal setting based on the selector in the future, but first guests only.

[Moini]

How can I turn this off? For some reason, nextcloud switched to dark mode today after I logged in, and I really dislike it. The dashboard is unreadable and everything looks unsharp to my eyes. I need dark on white text, which is available, but not accessible to me, apparently.

[Moini]

Update: another logout - login cycle fixed it...