Have a dark mode for the viewer

[mliebelt]

Nowadays many people use by default a dark mode, and the pgn-viewer is not aware of that. That may result in an ugly display, when all content is showing on a dark background, but the moves (and may be other parts as well) are shown with a light background.

The following should be done:

• Have a media query to switch between light and dark mode.
• Decide if the switch should be done all the time, or only when an option is set. Maybe it is a good way to have that option, but by default have it set to 'use what is defined'.
• May be an additional option in the UI to switch between dark and light mode could be implemented.