Issue #2600012 by joelpittet, chipway: 'css' key in batch_set is no longer used

 *     'finished' functions, for instance if they don't reside in the main

 *     .module file. The path should be relative to base_path(), and thus should

 *     be built using drupal_get_path().

 *   - css: Array of paths to CSS files to be used on the progress page.

 *   - library: An array of batch-specific CSS and JS libraries.

 *   - url_options: options passed to the \Drupal\Core\Url object when

 *     constructing redirect URLs for the batch.

 *   - progressive: A Boolean that indicates whether or not the batch needs to

      'init_message' => t('Initializing.'),

      'progress_message' => t('Completed @current of @total.'),

      'error_message' => t('An error has occurred.'),

      'css' => array(),

    );

    $batch_set = $init + $batch_definition + $defaults;