Support themes in code blocks in content editor

What does this MR do and why?

Currently only dark theme and light theme (default) is supported. More coming soon.

Issue: #334198 (closed)

Screenshots or screen recordings

Dark theme

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.