Unfortunately, this is a little bit involved.
Discourse runs version 10.6.0 of highlight-js. Most of the extra themes missing right now - base16 - require version 11.0
The issue is that that version introduces some breaking changes and deprecations. Have a look here
Release Version 11 · highlightjs/highlight.js · GitHub
So, this means that the current options are going to stay the same for a while. First, the highlight-js version needs to be updated in tests-passed in Discourse. Then we’d need to wait for a stable release. Updating the themes (even the current ones) would put us in a bad state where it’s guesswork whether the theme works properly on your site or not.