How to add classes and ids to main and secondary menu in Drupal 8. Add this to your theme's .theme file (in this case it's busy.theme). The .theme file is called template.php in Drupal 7.

gistfile1.txt

/**
 * Implements hook_preprocess_HOOK() for page.tpl.php
 */
function busy_preprocess_page(&$variables) {
  // Pass the main menu and secondary menu to the template as render arrays.
  if (!empty($variables['main_menu'])) {
    $variables['main_menu']['#attributes']['id'] = 'main-menu';
    $variables['main_menu']['#attributes']['class'] = array('links', 'clearfix');
  }
  if (!empty($variables['secondary_menu'])) {
    $variables['secondary_menu']['#attributes']['id'] = 'secondary-menu';
  }
}