Add Dark mode, fix #1

[1peter10]

Add dark mode, the very simple way.
Based on https://jsfiddle.net/vinorodrigues/qyx5o6tv/

[dbeley]

Thanks for this, were you able to test it?
On my side with firefox I wasn't able to properly trigger the dark mode with the dev-tools colorscheme simulation.
If it works on your side I will merge the PR anyway 👍

[1peter10]

I only tested it by adding the two lines to a downloaded copy of docs/index.html and it then worked in my firefox with dark theme. I will test again to confirm that it works and report back.

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