[WorldBuilder] Add dark mode theme to WorldBuilder

[tintinhamans]

Most/all new modern apps contain a dark mode feature; this should also be available in WorldBuilder as it is considered easier on the eyes.

[tintinhamans]

Very rough concept:

[seacat17]

If I'll ever use it, I'd use it in dark mode. I work at night, I don't want to burn my retinas.