Dark mode support in themes

[esk33]

Add support for a corresponding dark theme in themes, used when the user prefers dark theme via their browser/system.

[Lominub44]

With the normal colors this would look very shity, there would probably be many colors needed to change.
maybe its just me...

[esk33]

The whole point of a theme is that the colors are changed to not look "shitty" so I don't really see what your point is