pygments css files are generated via command line (from root):
pygmentize -S themename -f html -a .highlight >docs/extra/themename.css

Theme names "emacs" and "friendly" are okay in both light/dark modes.
They can be added to material.mkdocs.yaml as a single file.

However, per https://pygments.org/styles/,
"those styles might be "difficult to read for people with suboptimal vision".

As an alternative,
we can generate a light format with sufficient contrast using "default" style,
and a dark format with sufficient contrast using "lightbulb" format.
I then need to wrap the default css with:
[data-md-color-scheme="default"] {...rest of member...}
Likewise for the lightbulb css:
[data-md-color-scheme="slate"] {...rest of member...}
You can then add both files to material.mkdocs.yaml.