add support for browser color preferences

[ggvbo]

Sets the default value for Dark Mode depending on the browser's prefers-color-scheme media query, also listening to changes in the browser's media query.

[MiniDigger]

mmmh, I dont like this solution, we already have code for this in useSettingsHelper, but somehow that doesn't work as expected
edit: I think the db has a default value, thats why the thing doesnt work.
maybe we should only save to db if the user actually uses the button? else just rely on the media query?
but that would mean the server would serve stuff in the wrong color, which is meh too....