Add dark mode to processing cpp api reference

[TheBricktop]

Its mostly white, yellow accents and thin dark grey and grey fonts, and its eye-melting. It would be very much appreciated if there could be a dark mode or low light mode.

[pepc84]

Thanks for opening this, really appreciate the feedback! Currently working on it. Added a button in the top right that lets you permanently enable dark mode. Still finishing it up but it's mostly done.