Add css class to body with node type

/**
* Implements hook_preprocess_HOOK().
*/
function MYTHEME_preprocess_html(&$variables) {
 /** @var \Drupal\node\Entity\NodeType $node_type_entity */
 $node_type_entity = \Drupal::routeMatch()->getParameter('node_type');
 if ($node_type_entity) {
   $variables['attributes']['class'][] = 'node_' . $node_type_entity->id();
 }
}