Fix modal footer background color

What does this MR do and why?

The "dark mode" bg color for modals in gitlab-ui recently got updated and doesn't yet play nice with our current dark mode.

This reverts that color change to the modal's footer.

Changelog: fixed

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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.