Update main.js

Sorry, I misunderstood the meaning of "background === 'light'" as whether DarkenTTV was off; now I know it means that the color would *look best* on a light background.

I changed the lightening algorithm again, to something closer to what was originally intended; for colors to be lightened, instead of lightening by 20%, I inverted luminosity, darkened by 80%, then inverted again, so even black and blue would become light enough.