Ensure syntax highlighting with highlight.js matches dark theme

What does this MR do and why?

Ensures syntax highlighting with highlight.js matches our dark theme

Note: changes in this MR are behind the highlight_js feature flag.

Related to #345807 (closed)

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Change your syntax highlighting theme in User Preferences to the Dark theme
2. Enable the highlight_js feature flag
3. Open any source file

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.