Use different colors in light mode vs. dark

[garyo]

In a white-background terminal, it can be very hard to see some of the randomly-generated branch colors: It would be great if ogl could only pick "darkish" colors (luma < 0.8, for example) in light mode. Similarly, only pick lightish colors (luma > 0.3) in dark mode. Or maybe just always constrain the luma to 0.3..0.8 if that's easier.