Flarum comes natively with support for highlightjs since many versions. This extension might have offereded more easily accessible options but the theme can also be customized with the built-in formatter
https://s9etextformatter.readthedocs.io/Plugins/BBCodes/Add_from_the_repository/#code
I don't think the options mentioned in the documentation are accessible via bbcode, but those internal variables should be customizable by using our formatter extender. I don't think anyone has ever published the working code for this but it shouldn't be too complicated to change the default code theme in Flarum with just a few lines in extend.php.
