There is also a dark theme in draw.io for those people who prefer to work with white text/outlines on a dark background.
Still need to get work done and have a headache? Or perhaps you are diagramming in bed on your tablet and you don’t want to keep your partner awake with the bright, glowing screen? Instead of making everything extremely hard to read by simply dimming the screen, click Extras > Theme > Dark and reload the page to craft your diagram on a black background.
Be careful though – exported diagrams to image formats will also be on a black background if you haven’t or can’t set the background as transparent!