Fixed explain code button background color

What does this MR do and why?

Fixes the background color of explain code icon

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Enable the following feature flags:
   • explain_code
   • explain_code_snippet
   • openai_experimentation
   • ai_experimentation_api
2. Open a file containing Source Code (Note, this file should not be from the GitLab codebase).
   • sample_file.js
3. Highlight some of the Source Code.
4. Observe the (?) button

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.

Related to #407285