Dark mode implementation
Created by: Masboes
This implements the dark mode option in the general settings. It works by adding or removing the dark-mode class to the body element. This class is defined in the dark mode scss module and contains the css for each of the elements whose background and text have to change. All colors in use are variables declared at the top, so expanding the css for new elements on the page is easy.
fixes #112 (closed) fixes #102 (closed)