Codeblocks are not readable in dark mode

[nitya]

Action Taken:

1. Visit any page with a code block. For instance go to Concept - 1.1 Prompt Specifcation Section - and click to expand so the code block is visible.
2. Switch page toggle to enforce light mode - the content is readable (black text on white background)
3. Switch page toggle to enforce dark mode - the content is no longer readable (white text on light gray)

Expected:
Background to be a color that offered sufficient contrast against default white text.

Why it matters:
Explainers may reference code snippets with line numbers so visbility helps
The copy-paste feature is not impacted.