As suggested by @Sirko in the comments of the question, the solution in this case is to include one of the highlightjs CSS files in the resulting markup.

The docs are a bit sparse on the matter, but a full example can be found in their README (and is copied here):

<link rel="stylesheet" href="/path/to/styles/default.min.css">
<script src="/path/to/highlight.min.js"></script>
<script>hljs.highlightAll();</script>

(note specifically the stylesheet <link>.)