Correct background in input fields and footer in login pages

What does this MR do and why?

When focused or active, the background of input fields was set to white. That interfered with dark mode.

Adapt the background color for the footer.

Closes #414970 (closed)

Changelog: changed

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable dark mode for the app by changing default_theme: 11 in config/gitlab.yml
2. Restart gdk
3. On an incognito window sign in: https://gdk.test:3443/users/sign_in

MR acceptance checklist

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