Customizer widgets: Add ThemeProvider for admin color schemes

[mirka]

What?

Wraps the Customizer widgets editor in a root ThemeProvider, seeded with the active admin color scheme primary color.

Follow up to #81112

Why?

UI in the Customizer widgets block editor need a ThemeProvider ancestor to pick up design token overrides. The post editor and Site Editor already do this at their layout layers; the Customizer widgets editor did not.

The Customizer widgets UI uses white/near-white surfaces, so only the admin primary is seeded. Background tokens fall back to the design system default (#fcfcfc). isRoot forwards tokens to the document element so portaled UI can inherit them.

Testing Instructions

1. Start wp-env and build the branch (npm start).
2. Activate a classic theme with widget areas (e.g. Twenty Twenty).
3. Open the Customizer (/wp-admin/customize.php).
4. Expand Widgets and open a widget area.
5. Switch admin color schemes under Users → Profile (e.g. Ocean, Midnight, Sunrise, Light).
6. Reload the Customizer and confirm WPDS-styled UI reflects the chosen scheme's primary/accent colors with readable contrast on white surfaces.

[github-actions]

Size Change: +301 B (0%)

Total Size: 7.81 MB

📦 View Changed

Filename	Size	Change
build/scripts/customize-widgets/index.min.js	14.9 kB	+301 B (+2.06%)

_{compressed-size-action}

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: mirka <0mirka00@git.wordpress.org>
Co-authored-by: aduth <aduth@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[aduth]

This makes sense to me and the implementation follows what I'd expect 👍

I wasn't positive what I should be looking for in testing, but I did observe that the focus ring for the menu here changed from the default blue to the green following my color scheme (Ectoplasm).

[github-actions]

I just cherry-picked this PR to the wp/7.1 branch to get it included in the next release: e3743be