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 in DarkenTTV); this changed algorithm converts to HSL, performs lightening or darkening *only* on colors that are too dark (or light, respectively), without changing hue at all, and converts back to RGB (that is, clipping rather than changing everyone's chat color).

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; my "lim" variable could be changed according to taste, but should be well above 0 and below 0.5, or else Blue won't be lightened at all (and Yellow won't be darkened at all).