macOS's terminals default to a white background, but don't convey that

fact in any environment variables. However, it does support the ANSI

escape sequence used to fetch the color palette.
We now fall back to invoking this ANSI escape sequence, attempting to

fetch the background color. This comes back as a XParseColor value in

the form of 11;rgb:RRRR/GGGG/BBBB, where the red/green/blue are values

between 1 and 4 characters in length, as hex values representing a

fraction of the color in the available color depth.
These are converted back to traditional RGB values between 0-255. We

then calculate a luma value from this, and use that to determine if we

want to show dark vs. light colors. This ends up working fairly well for

most any realistic color palette, and has been tested across all kinds

of backgrounds.
The header text has also been changed to work better on many lighter

color backgrounds. Rather than reverse text, we now use an explicit

black on green, which should be stable across all "light" backgrounds.

Tested this across a wide range of backgrounds. I tested white, black,

a spectrum of shades of grey, various blues, reds, yellows, greens, and

some combinations of those.
Tested without a TTY.
Tested this on macOS (Apple Terminal and iTerm) and Linux terminals.