Issue #3123296 by VladimirAus: Error copying new subtheme via interface and enabling it

_SUBTHEME/SUBTHEME.info

@@ -4,9 +4,13 @@ description: 'SUBTHEME subtheme'
 core: 8.x
 base theme: bootstrap4
 
+# Stylesheets.
+stylesheets-remove:
+  - '@bootstrap4/css/styles.css'
+
 # Libraries.
-libraries-override:
-  bootstrap4/global-styling: SUBTHEME/global-styling
+libraries:
+  - SUBTHEME/global-styling
 
 # Regions.
 regions:

bootstrap4.theme

@@ -202,7 +202,7 @@ function bootstrap4_form_system_theme_settings_subtheme_validate(array &$form, F
   // Check for writable path.
   $directory = DRUPAL_ROOT . '/' . $form_state->getValue('subtheme_folder');
   if (\Drupal::service('file_system')->prepareDirectory($directory, FileSystemInterface::CREATE_DIRECTORY | FileSystemInterface::MODIFY_PERMISSIONS) === FALSE) {
-    $form_state->setErrorByName('subtheme_folder', t('Subtheme cannot be created.'));
+    $form_state->setErrorByName('subtheme_folder', t('Subtheme cannot be created. Check permissions.'));
   }
   // Check for common theme names.
   if (in_array($form_state->getValue('subtheme_machine_name'), [
@@ -237,6 +237,9 @@ function bootstrap4_form_system_theme_settings_subtheme_submit($form, FormStateI
   $themeMName = $form_state->getValue('subtheme_machine_name');
   $themePath = $drupalRoot . '/' . $form_state->getValue('subtheme_folder') . '/' . $themeMName;
   $fs->mirror(drupal_get_path('theme', 'bootstrap4') . '/_SUBTHEME', $themePath);
+  // Copy CSS file replace empty one.
+  // Will be replaced after SCSS compilation is done.
+  $fs->copy(drupal_get_path('theme', 'bootstrap4') . '/css/style.css', $themePath . '/css/style.css', TRUE);
 
   $files = [
     'SUBTHEME.info' => $themeMName . '.info.yml',
@@ -248,7 +251,8 @@ function bootstrap4_form_system_theme_settings_subtheme_submit($form, FormStateI
       $infoYml = Yaml::decode(file_get_contents($themePath . '/' . $before));
       $infoYml['name'] = $themeName;
       $infoYml['description'] = $themeName . ' subtheme based on Bootstrap 4 theme.';
-      $infoYml['libraries-override']['bootstrap4/global-styling'] = $themeMName . '/global-styling';
+      $infoYml['libraries'] = [];
+      $infoYml['libraries'][] = $themeMName . '/global-styling';
       $fs->dumpFile($themePath . '/' . $after, Yaml::encode($infoYml));
     }
     else {