Every basic website built with Jekyll will use the Rouge gem to highlight code syntax. You can customise the look and feel by adding any of these themes to _sass/_syntax.scss.
Also check you _config.yml for the following options:
markdown: kramdown
kramdown:
syntax_highlighting: rouge
highlighter: rouge
For a full example, see my website's source code: https://gitlab.com/protesilaos/protesilaos.gitlab.io