Login and Registration: Ensure language switcher button remains readable

[ArkaPrabhaChowdhury]

The login language switcher uses the default secondary button style, which now has a transparent background. On customized login screens with dark backgrounds, this can leave the button text and border without sufficient visual separation.

This adds the standard light admin background color specifically to the language switcher button so it remains readable independently of the login page background.

Trac ticket: https://core.trac.wordpress.org/ticket/66130

Testing

1. Install an additional language so the language switcher appears on wp-login.php.
2. Set the login page background to #3c3c3c.
3. Confirm that the language switcher button has a #f6f7f7 background and remains readable.

Browser verification confirmed the button background changes from transparent before the patch to rgb(246, 247, 247) after the patch, while retaining the existing text and border colors.

Checks run:

• grunt precommit:css
• php vendor/bin/phpcbf src/wp-login.php
• php vendor/bin/phpcs src/wp-login.php
• git diff --check

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5
Used for: Ticket analysis, implementation, browser verification, test execution, and drafting this pull request description.

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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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