Change chat background color and chat bubble color

What does this MR do and why?

This MR changes the background color and the bubble color of the GitLab Duo chat.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

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

MR acceptance checklist

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

Related to #419286 (closed)