Dark mode styles

[Malvoz]

Some people prefer dark mode. IIRC Amelia added dark mode styles to maps4html.org. And web-map-docs has dark mode styles from Docusaurus. We should add dark mode styles to other repos.

• Dark mode styles for specs that use ReSpec (UCR and MapML):
  W3C recently deployed dark mode styles. But I think we should wait for ReSpec to integrate them because adding them manually seem to cause a flash of dark mode styles for light mode users.

• Dark mode styles for other reports and repos (UCR-MapML-Matrix, Experiments, Web-Map-Custom-Element):
  Add a small custom style sheet with basic styles such as background, text- and link color (perhaps based on the W3C dark mode styles). It can probably be a shared style sheet hosted from https://github.com/Maps4HTML/Maps4HTML.github.io/tree/master/assets (i.e. maps4html.org/assets/dark.css). And if needed we can add/override styles in any particular repo (e.g. tables in UCR-MapML-Matrix).