Habilita las sugerencias para aplicar plantillas en bloques

/**
* Implements template_theme_suggestions_block_alter().
*/

function mitema_theme_suggestions_block_alter(&$suggestions, $variables) {
    $content = $variables['elements']['content'];
       if (isset($content['#block_content']) && $content['#block_content'] instanceof \Drupal\block_content\BlockContentInterface) {
           $suggestions[] = 'block__' . $content['#block_content']->bundle();
   }
}