add support for 'prefers-color-scheme' for dark mode / light mode browser preference detection

[foss-]

Nextcloud supports dark and light mode in User > Settings > Accessibility.

1. This is a bit unexpected as dark / light mode is not really an accessibility feature and in most software found under design or general settings.
2. Discoverability is not great. I am using dark mode on macOS which results in Firefox following that system preference (as expected). As Firefox supports prefers-color-scheme I would expect NextCloud to pick up on that and follow the user preference.

This would result in a UI which integrates nicely with the rest of the system the user is using. Fairly low hanging fruit with nice impact.

[szaimen]

Duplicate of #12276