Fix: Added style for wp-activate.php and wp-signup.php for gap between header/footer and screen.

[umeshnevase]

• I've enabled Network site settings for Both site and user registration on multisite WordPress.
• There are no space between screen and header/footer.
• The issue is coming for all FSE themes 2022, 2023, 2024, 2025. It looks find theme 2021 and before.
• I've added filter for body classes for both the pages. and added style by using those new classes in this PR.
• We have handle this CSS in theme, there are more chance for improvement for UI on both these pages

(Trac #63499)

[github-actions]

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Props umeshnevase.

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[github-actions]

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