function template_preprocess_entity(&$vars, $hook) {
  
  // Add the View Mode class to the classes_array
  if (isset($vars['elements']['#entity_view_mode']['view_mode'])) {
    $view_mode = $vars['elements']['#entity_view_mode']['view_mode'];
    $vars['classes_array'][] = 'view-mode-' . str_replace('_', '-', $view_mode);
  }
  
}