This combines ansi-light and ansi-dark into a single theme that works
with both light and dark backgrounds. Instead of specifying white/black,
the ansi theme uses the terminal's default foreground/background color
by setting alpha=01, i.e. #00000001. This is in addition to the alpha=00
encoding where red contains an ANSI color palette number.
Now, `--theme ansi-light` and `--theme ansi-dark` will print a
deprecation notice and use ansi instead (unless the user has a custom
theme named ansi-light or ansi-dark, which would take precedence).
