Here are more resources I’ve found, but haven’t sat down yet to put together.
https://docs.asciidoctor.org/pdf-converter/latest/theme/apply-theme/
https://jekyll-one-org.github.io/pages/public/skeleton/documentation/200_themes/
https://github.com/asciidoctor/asciidoctor-pdf/tree/main/data/themes
https://jekyll.one/pages/public/manuals/components/j1/plugins/asciidoctor/ (not really about theming, but looks interesting)
John Poelstra 