Add Darkmode

[a-shel]

I managed to make a kind-of darkmode manually by tweaking the css files with the inspect tool. Of course it needs to be saved and to eventually replace the "vanilla" one in other exports, but no hassles after the first time.

(but a clean darkmode button would be much nicer)

[christopher-besch]

What you can also do is to install Darkreader. I programmed the Manim Editor to (unofficially) look good with it.