Add the View Mode to the classes_array within your entity template. This is so we can theme specific view modes so we can easily switch between different layouts for the same entity using entity view modes

template.php

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);
  }
  
}