When the code highlighter filter is enabled, the text within code blocks can have a poor colour contrast against the greyish background.

To reproduce:

1. Log in as admin
2. Go to Site administration > Plugins > Filters > Manage filters
3. Enable the Code highlighter filter.
4. Restore the example course backup CodeExamplesBackup.mbz.
5. Go to the restored course
6. Run axe DevTools on the page.
7. Expected: There are no errors related to colour contrast.
8. Actual: There's a bunch of errors related to colour contrast.