Fix: Disable Custom CSS command for non block themes

[DarkMatter-999]

What?

Closes #73408

Hide the "Open Custom CSS" command from the Command Palette when the active theme is not a block theme.

Why?

The "Open Custom CSS" command is currently available when using hybrid themes (themes with theme.json but not full block themes). However, invoking this command navigates to the Site Editor and results in an error:

"You attempted to edit an item that doesn't exist. Perhaps it was deleted?"

This happens because custom CSS editing via Global Styles is only supported for block themes. Hybrid themes may expose partial data, which causes the command to appear even though the feature is not actually usable.

This leads to a broken user experience where users can access a command that always fails.

How?

This PR adds an additional guard to the command registration logic:

• Uses getCurrentTheme() from the core store to determine whether the active theme is a block theme (is_block_theme)
• Ensures the "Open Custom CSS" command is only registered when the active theme is a block theme (isBlockBasedTheme)

Testing Instructions

1. Use a site with a hybrid theme (classic theme with theme.json, but not a block theme)
2. Open the Command Palette
3. Search for "Custom CSS"

Expected:

• The command should not appear

4. Switch to a block theme
5. Open the Command Palette again
6. Search for "Custom CSS"

Expected:

• The command appears
• Clicking it opens the Global Styles CSS editor without errors

Screenshots or screencast

Before

custom-css-hybird-theme.mov

After

custom-css-fix.mov

Use of AI Tools

Drafted PR description with assistance from ChatGPT. The implementation and final PR content were reviewed and validated manually.

P.S.
An alternative approach could be to redirect users on hybrid and classic themes to the Customizer’s "Additional CSS" panel instead of hiding the command entirely as in this previous PR(#65155). This would require confirming that the user has permission to edit theme options, which might be approximated in JS via:

select( 'core' ).canUser( 'update', 'themes' );

[github-actions]

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Co-authored-by: DarkMatter-999 <lakshyajeet@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: MadtownLems <madtownlems@git.wordpress.org>

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[t-hamano]

LGTM! For now, this approach seems to be the simplest 👍