add Constant Perceptual Luminosity (dark) theme,

[aaronchall]

Optimized for color differences subject to constant luminosity.

blurb:
ANSI term colors picked with OKHSL maximizing difference
in color subject to constant perceptual luminosity so that all
colors are readable on dark screens even against transparency
yet gentle on the eyes in dark environments.
for colors (starting with red at 30 degrees) luminosity is .4, .6 (bright)
.5 for white(gray), .7 bright white (light gray)
0 for black and .3 for bright black
For transparency I find these work ok:

background_opacity 0.7
dynamic_background_opacity yes