Admin reskin: Add fallback values to the CSS custom properties for the media view and button styles

[t-hamano]

Trac ticket: https://core.trac.wordpress.org/ticket/64840

Use of AI Tools

All code changes were made by Copilot, and I have verified that all changes are correct.

Testing instructions

One way to test this PR is to use Elementor. Elementor uses media modals on screens that are not enqueuing CSS custom properties.

1. Install Elementor
2. Open http://localhost:8889/wp-admin/post-new.php
3. Click the " Edit with Elementor" button (If it's not enabled)
4. Click the Image button inside the left sidebar
5. Click the "Choose Image" button
6. Confirm the button colors

Screenshot

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[github-actions]

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Props wildworks, shailu25.

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[github-actions]

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[shail-mehta]

The fallback value is currently being applied to the Media View button. ✅

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