Adding a class to a region
What I normally do for all my themes is to add the missing region region-name class to each region by implementing hook_preproccess_HOOK in MYTHEME.theme.

/**
 * Implements template_preprocess_region().
 */
function MYTHEME_preprocess_region(&amp;$variables) {

  // Add missing region CSS classes.
  $variables['attributes']['class'][] = 'region';

  if (isset($variables['region']) &amp;&amp; !empty($variables['region'])) {

    $variables['attributes']['class'][] = 'region-' . str_replace('_', '-', $variables['region']);
  }
}