Stylesheets-override has been removed

Stylesheets-override has been removed. hook_library_info_alter() can be used as an alternative. This was necessary to fix a critical issue in the theme system. There is an issue to bring this functionality back using libraries #2451411: Add libraries-override to themes' *.info.yml. Also, see this change record for the change in how stylesheets-remove works. https://www.drupal.org/node/2473869

Themers were able to override stylesheets on themename.info.yml file:

stylesheets-override:
  - vertical-tabs.css

but now this has to be done in themename.theme file:

/**
 * Implements hook_library_info_alter().
 */
function themename_library_info_alter(&$libraries) {
  if (isset($libraries['drupal.vertical-tabs'])) {
    unset($libraries['drupal.vertical-tabs']['css']['component']['misc/vertical-tabs.css']);
    $libraries['drupal.vertical-tabs']['dependencies'][] = 'themename/vertical-tabs';
  }
}

Comforter kitten: