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  // Create a dedicated attributes array for the HTML element.

  // By default, core does not provide a way to target the HTML element.

  // The only arrays currently available technically belong to the BODY element.

  // Create a dedicated attributes array for the BODY element.

  if (!isset($variables['body_attributes_array'])) {

    $variables['body_attributes_array'] = array();

  }

  // Ensure there is at least a class array.

  if (!isset($variables['body_attributes_array']['class'])) {

    $variables['body_attributes_array']['class'] = array();

  }

 * **WARNING**: It is not recommended that this function be copied to a

 * sub-theme. There is rarely any need to process the same variables twice.

 *

 * If you need to add something to the "html_attributes_array" or

 * "body_attributes_array" arrays, you should do so in a hook_preprocess_html()

 * function since process functions will always run after all preprocess

 * functions have been executed.

 *

 * If there is a need to implement a hook_process_html() function in your

 * sub-theme (to process your own custom variables), ensure that it doesn't

 * add this base theme's logic and risk introducing breakage and performance

 * issues.

 *

  // Merge in (not reference!) core's ambiguous and separate "attribute" and

  // "class" arrays. These arrays are meant for the BODY element, but it must

  // be done at the process level in case sub-themes wish to add classes to

  // core's non-standard arrays (which are for the BODY element only).

  // @see https://www.drupal.org/node/2868426

  $variables['body_attributes_array'] = drupal_array_merge_deep($variables['body_attributes_array'], $variables['attributes_array']);

  // Use this project's class helper (to eliminate any duplicate classes).

  _bootstrap_add_class($variables['classes_array'], $variables, 'body_attributes_array');

  // Finally, convert the arrays into proper attribute strings.