Dark mode

[fidoboy]

¿dark mode support?

[amnuts]

@fidoboy, I like the idea and it's a possibility, but some people embed the gui in other systems, such as Moodle or Pimcore, which then might react weirdly to the colour scheme being set on the whole page. But I'll have a play around and see how it shakes out.

[amnuts]

I've started in on this, but haven't sorted out the colours just yet. I also need to sort out saving the toggle state to the browser's cookie or something. So it's not ready yet, but if you want to follow along, it's at #108