Code snippet that can be used to add a CSS class to a form label in Drupal 8.
/**
* @param $variables
*/
function THEMENAME_preprocess_form_element_label(&$variables)
{
if (isset($variables['element']['#id'])) {
if ($variables['element']['#id'] == "ELEMENT_ID") {
$variables['attributes']['class'][] = 'form-label-class';
}
}
}
