The github-markdown.css file includes syntax highlighting style rules, e.g.:
https://github.com/sindresorhus/github-markdown-css/blob/gh-pages/github-markdown.css#L523

However, it's not clear how to set these classes when generating HTML from Markdown. It would be helpful to provide an explanation of how to do so. (In particular, is there a way to configure marked to set these classes?)

PS: Thanks for writing this great module! I'm using it here: https://github.com/asadovsky/mdviewer