How can I disable CSS styles from a core module using libraries-override?

Question

I'm trying to disable the CSS styles from the layout discovery module in my theme.info.yml file. I've tried several options, but I can't seem to get it to work. This is what I have now.

libraries-override:
  core/layout_discovery:
    css:
      theme:
        core/modules/layout_discovery/layouts/twocol/twocol.css: false

The CSS styles can be removed using stylesheets-remove at the moment, but that property has been deprecated. It does seem work at the moment, though.

I would like to find the proper way to remove the CSS stules using libraries-override, if I can.

stylesheets-remove:
  - core/modules/layout_discovery/layouts/onecol/onecol.css

Answer 1

Seems you are missing a starting / on the full path. And the name of the overriding library looks wrong as well. There's no library core/layout_discovery. It must be layout_discovery/twocol instead.

libraries-override:
  layout_discovery/twocol:
    css:
      theme:
        /core/modules/layout_discovery/layouts/twocol/twocol.css: false

Source: Overriding and extending libraries

---

Alternatively, unsetting libraries via hook_page_attachments_alter() does the trick as well. Here comes a working code snippet to be placed in your theme's *.theme file. Replacy MYTHEME with the machine name of your theme.

/**
 * Implements hook_page_attachments_alter().
 */
function MYTHEME_page_attachments_alter(array &$attachments) {

  foreach ($attachments['#attached']['library'] as $key => $library) {

    // Install the Devel submodule Kint for debugging.
    // Check all libs:
    // ksm($attachments['#attached']['library']);

    // Here I disable the core base library.
    // Looks pretty ugly afterwards.
    if ($library == 'system/base') {
      unset($attachments['#attached']['library'][$key]);
    }

    // So for you it must be like following.
    // Simply try to find the right name via ksm($library);
    // or the other ksm() command from above.
    if ($library == 'layout_discovery/twocol') {
      unset($attachments['#attached']['library'][$key]);
    }
  }
}

Answer 2

I wanted to reduce count of http-requests and unset all unnecessary Drupal core css-files. Finally, this configuration works:

libraries-override:    
  layout_discovery/onecol: false

It should be added in file MY_THEME_NAME.info.yml
Do not forget "clear all caches" on 'admin/config/development/performance' page.

Answer 3

It was difficult to identify the correct library to exclude, using hook_css_alter made it easy for me to exclude a specific file.

function THEME_css_alter(&$css) {
  $cssfile= "core/themes/stable/css/core/vertical-tabs.css";
  if ($css[$cssfile]) {
    unset($css[$cssfile]);
  }
}