Update main.js

In general, the lightening/darkening algorithm in BTTV does change the hue of a color, particularly when one of the RGB components is FF, another is 00, and the third is neither (for example, a deep purple, with small nonzero R, G=0, and B=FF, will be hue-shifted toward magenta); this changed algorithm converts to HSL, performs lightening or darkening *only* on colors that are too dark (or light, respectively), and converts back to RGB.

Based on my observations, I suspect that "too dark" for DarkenTTV is below 64% luminosity, and "too light" for the default theme is above 36% luminosity.