It seems that
https://cdn.jsdelivr.net/gh/highlightjs/cdn-release@11.0.0/build/styles/github-gist.min.css
just return 404
Can't get syntax highlight correctly
stimw Maybe because of Internet Outage?
- Edited
stimw its a known bug, will be fixed in the upcoming release (i don't know precisely when)
Justoverclock
I'm surprised that it was not fixed in 1.0.3 
stimw developers are working hard every day, trust me will be fixed soon 
Sure, thanks to the contributors for their work.
21 days later
- Edited
- Best Answerset by stimw
This issue should be fixed by updating your forum dependencies. It was an issue in another project that Flarum uses to perform formatting on posts.
@JoshyPHP released an update to s9e/TextFormatter which switched out the broken CSS for a different one a few days ago: https://github.com/s9e/TextFormatter/releases/tag/2.9.4