Allow overriding/disabling base theme's ckeditor5-stylesheets value

Problem/Motivation

Is there a straightforward way to override/disable the base theme's ckeditor5-stylesheets? Base theme's CKEditor5 stylesheet gets loaded by _ckeditor5_theme_css() regardless of my sub-theme's ckeditor5-stylesheets value.

I am using bootstrap5 as base theme. It has the following lines and seems to follow the instructions in Change record: New API for adding theme-specific styles in CKEditor 5 → .

ckeditor5-stylesheets:
  - css/ck5style.css

This issue happens in both D10 and D11 with any base theme that has CKEditor5 stylesheets.

The change record → above does not say anything about the expected behaviour regarding the base theme's CKEditor5 stylesheets. So, I'm not sure if this should be considered a feature request or a bug.

Workaround: Here is a workaround, but it would be easier to control this through sub-theme's ckeditor5-stylesheets value.

/**
 * Implements hook_system_info_alter().
 */
function mymodule_system_info_alter(array &$info, Extension $file, $type) {
  if ($type == 'theme' && $file->getName() == 'bootstrap5') {
    $info['ckeditor5-stylesheets'] = FALSE;
  }
}

Steps to reproduce

0. Make sure ckeditor5 is installed.

1. Install a base theme that has ckeditor5-stylesheets value, for example bootstrap5 → (inside D10, as bootstrap5 is not available in D11 yet).

composer require 'drupal/bootstrap5:^4.0'
drush theme:install bootstrap5

2. Create a sub-theme. For bootstrap5, you can use the following command:

drush --include="web/themes/contrib/bootstrap5/src/Drush" bootstrap5:generate-subtheme my_subtheme --subtheme-name="My sub-theme"

3. Observe that our new sub-theme has the following lines and seems to follow the instructions in the change record → above:

...
ckeditor5-stylesheets:
  - css/ck5style.css
...
base theme: bootstrap5

4. Install the sub-theme and make it the default theme. Also, turn off CSS aggregation for easier testing and clear cache.

drush theme:install my_subtheme
drush config-set system.theme default my_subtheme
drush config-set system.performance css.preprocess 0
drush cr

5. Test through browser/drush:

5.a Test in browser: Go to /node/add/page and observe that CKEditor5 stylesheets are loaded for both base theme and sub-theme (for example, the /themes/contrib/bootstrap5/css/ck5style.css and /themes/custom/my_subtheme/css/ck5style.css).

5.b Check _ckeditor5_theme_css()'s output by executing the following drush command:

drush eval 'echo var_dump(_ckeditor5_theme_css());'

... and observe that CKEditor5 stylesheet of base theme still shows up in the output in addition to the sub-theme's:

array(2) {
  [0]=>
  string(43) "/themes/contrib/bootstrap5/css/ck5style.css"
  [1]=>
  string(48) "/themes/custom/my_subtheme/css/ck5style.css"
}

6. Now set ckeditor5-stylesheets to false in your sub-theme's .info.yml file:

...
ckeditor5-stylesheets: false
...
base theme: bootstrap5

7. Clear cache and repeat the step 5 above for testing:

7.a Test in browser: Observe that base theme's CKEditor5 stylesheet (for example, the /themes/contrib/bootstrap5/css/ck5style.css above) is loaded.

7.b Check _ckeditor5_theme_css()'s output using the following command:

drush eval 'echo var_dump(_ckeditor5_theme_css());'

and observe that the base theme's CKEditor5 stylesheet still shows up in the output:

array(1) {
  [0]=>
  string(43) "/themes/contrib/bootstrap5/css/ck5style.css"
}

Proposed resolution

Either of the following:

• Load the base theme's CKEditor5 stylesheets only if the sub-theme does not provide it (this behaviour is mentioned by alex.skrypnyk in the change record page → .) This is the easiest option I think, as we just need to stop looking once we find a theme (the sub-theme or a base theme) that has ckeditor5-stylesheets set (either false or actual CKEditor5 stylesheets). However, it may cause issues for existing sites that are intended to use CKEditor5 stylesheets from both base theme and the sub-theme.
• Provide a functionality similar to libraries-override.