Hakyll has built-in support for syntax highlighting thanks to its use of Pandoc. Pandoc itself uses highlighting-kate, which has a well-defined interface for customizing the highlighting colors used.

On the GitHub page page for highlighting-kate you’ll find a list of languages for which syntax highlighting is supported. It also has instructions for getting a pre-existing set of syntax highlighting CSS files.

See Ben Jeffrey’s blog for an article on creating your own custom CSS for syntax highlighting for Pandoc output.