Custom Colors: Fix block editor CSS in Gutenberg 22.6.0+ iframe context

[arthur791004]

Related to THEME-132

Proposed changes

• Add a dedicated print_block_editor_css() method to Colors_Manager_Common that adapts Custom Colors CSS selectors for the block editor iframe context introduced in Gutenberg 22.6.0+.
• Update the enqueue_block_editor_assets hook to use the new method instead of print_theme_css().
• The new method extends #editor .editor-styles-wrapper selectors to also match :root :where(.editor-styles-wrapper) inside the iframe, while preserving the original selector for backwards compatibility with older Gutenberg versions.
• Enqueue custom colors as an inline style via wp_add_inline_style() instead of printing a raw <style> tag, which is the correct approach for the enqueue_block_editor_assets hook.

Other information

• Generate changelog entries for this PR (using AI).

Related product discussion/links

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. Activate a theme that supports Custom Colors (e.g. Morden).
2. Set custom colors via Appearance → Customize → Colors & Backgrounds, especially the one with dark background
3. Open the block editor (post or page editor).
4. Before this patch: Custom color CSS does not apply inside the editor iframe on Gutenberg 22.6.0+.
5. After this patch: Custom color CSS correctly applies inside the editor iframe. The editor preview should reflect the custom color choices.
6. Verify that the frontend (wp_head) still renders custom colors correctly (no regression).

[github-actions]

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[github-actions]

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[arthur791004]

Sync changes from 207833-ghe-Automattic/wpcom

[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/wpcomsh/custom-colors/colors.php	0/842 (0.00%)	0.00%	11 💔

Full summary · PHP report

_{Coverage check overridden by Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR .}