Description of feature

It would make it easier on my eyes ;)

As far as I can tell most colours should still be 'OK'(tm), it would mainly be flipping the white background to black, and black text to white. Only additional thing would be that the you would need a dark-mode version of the MultiQC logo - something I'm not sure how easy it would be.

In addition, this could be supported by a toggle switch, and ideally a configuration file boolean.