D8-ckeditor-widget-styles.php

<?php
/**
 * Implements hook_editor_js_settings_alter().
 */
function sitefarm_core_editor_js_settings_alter(array &$settings) {
  // New Styles
  $styles = array(
    [
      'name' => 'Align Left',
      'type' => 'widget',
      'widget' => 'embed',
      'attributes' => ['class' => 'u-align--left u-width--half'],
    ],
    [
      'name' => 'Align Center',
      'type' => 'widget',
      'widget' => 'embed',
      'attributes' => ['class' => 'u-align--center u-width--two-thirds'],
    ],
    [
      'name' => 'Align Right',
      'type' => 'widget',
      'widget' => 'embed',
      'attributes' => ['class' => 'u-align--right u-width--half'],
    ],
  );

  foreach ($settings['editor']['formats'] as $name => $value) {
    // Add new styles
    $current_styles = $settings['editor']['formats'][$name]['editorSettings']['stylesSet'];

    if ($current_styles) {
      $settings['editor']['formats'][$name]['editorSettings']['stylesSet'] = array_merge($current_styles, $styles);
    }
  }
}