Hello, I'm new to AsciiDoc, and I seem to be having the same basic problem as the OP, except not in PDF.
I'm uploading AsciiDoc files to GitHub / GitLab, but I can't get colored text to display in the rendered HTML.
The project is available at https://github.com/youngjim01/asciidoc-test
I've also posted the same question on Stack Overflow at https://stackoverflow.com/questions/69368301/how-to-get-rendered-colored-text-in-gitlab-github-using-asciidoc
If anybody can point me in the right direction, I'd be very grateful.