Set the default background for notice-info to white

[joedolson]

Changes the core notice info to have a white background.

This doesn't match the Gutenberg design specs, but because WordPress uses a gray background, it's not actually possible to meet those specs 100%, and this will be a less disruptive change for users.

Trac ticket: https://core.trac.wordpress.org/ticket/64678

Use of AI Tools

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props joedolson, peterwilsoncc, pbiron.

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[github-actions]

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[peterwilsoncc]

@joedolson I've taken the liberty of pushing the same change to login.css to apply the background during the password reset flow, etc.

[peterwilsoncc]

I'm happy with Joe's changes if Joe is happy with mine.

I did a search for other instances of the string .notice searching for overrides in (S)CSS files and was unable to find one.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61737
GitHub commit: 17d91f6

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.