Remove custom CSS to fix modal background color

What does this MR do and why?

Modal background color changed in gitlab-ui recently in gitlab-ui!4314 (merged), but was hardcoded here for header and body with this custom CSS. But not for the footer, so header+body were using the old, and only footer the new bg color from gitlab-ui.

Changelog: fixed

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

• Check any modal that has header + body + footer. Their background color should be identical.