Adding a class to “body”
Alternatively, you could implement template_preprocess_html in your [themename].theme file.

function themename_preprocess_html(&amp;$variables) {
  $variables['attributes']['class'][] = 'class-name';
}


See the documentation: https://api.drupal.org/api/drupal/core%21includes%21theme.inc/function/template_preprocess_html/8.2.x