Implement proper Dark mode

[kelson42]

We have discussed the way how we want to implement content/ZIM dark mode, see https://github.com/openzim/overview/issues/53. The result of the discussion is that we want - at least - to support the open Web approach based on the CSS prefered-color-scheme.

That means we need now to implement/test (part of the things already work) the following points for both iOS and macOS:

• Implement a Setting for "Theme" with three values "System/Light/Dark". The first one been the default with the value coming from the OS, ant the two last ones forcing activation or desactivation. Should be mostly a relabelling of current settings.
• The content should strictly follow this settings, here is a demo ZIM file https://tmp.kiwix.org/light-dark.zim
• The UI should strictly follow this settings
• Any other system already potentially in place which changes the color sheme of the content should be removed.