Custom Colors: fix theme background color ignoring 'Use theme styles' toggle

[msilbers]

What

Fixes EDI-504 — theme background color displays in the editor when "Use theme styles" is disabled on classic themes (Atomic/WoA sites).

Companion PR to wpcom #218901 which fixes the same issue on Simple sites.

Why

Classic themes (Twenty Sixteen, Twenty Twenty, etc.) with custom background colors inject those colors into the block editor via wp_add_inline_style(), which bypasses Gutenberg's style management. The "Use theme styles" toggle has no effect on these injected styles.

How

Switches the CSS injection from enqueue_block_editor_assets + wp_add_inline_style() to the block_editor_settings_all filter with __unstableType => 'theme', so Gutenberg treats them as theme styles and the toggle controls them.

The existing print_block_editor_css() method is left in place as it may be called from other contexts or subclasses.

Testing

• On an Atomic site with Twenty Sixteen + custom background color:
  • "Use theme styles" ON → custom background shows in editor
  • "Use theme styles" OFF → custom background removed from editor
• Repeat with Twenty Twenty
• Verify published posts display correctly in both states
• Verify no regressions in editor color rendering for other classic themes

Note: Verified via code review and parity with the wpcom Simple site fix. Reviewer input on live-site behavior requested.

Note: The Linear issue was auto-marked "In Progress" when the wpcom PR opened. This is review-ready, not actively worked by an engineer.

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