For the HTML output you also have the choice of adding custom CSS.
I've answered a similar question on StackOverflow. Maybe this is close to what you are looking for:
https://stackoverflow.com/questions/49189563/sublist-numbering-in-asciidoc/57244921#57244921You can add CSS as either pass through content, or as a custom stylesheet that replaces the standard asciidoctor CSS stylesheet. See
https://asciidoctor.org/docs/user-manual/#applying-a-theme for more information.