Propose a dark theme

In order to leverrage system side user preferences, and allow a better experience for power users, we can implement a dark color theme.
As of now, in Administration, an administrator can choose ONE color theme for all pages. Implementing a dark color theme alternative will most likely involve:

• Adding a second option in administration to pick the dark alternative.
• Adding a `dark mode` preference in the user preference
• Choose to query compiled LESS from the light or dark color theme based on the xwiki user preference or `prefers-color-scheme` (MDN web doc->prefers-color-scheme)

This is responsiveness based on user preferences.