How to change active link css class (is-active)
You can override the theme_menu_link function like this:

function theme_menu_link($variables)
{
  $element = $variables['element'];
  $sub_menu = '';

  if ($element['#below']) {
    $sub_menu = \Drupal::service('renderer')-&gt;renderRoot($element['#below']);
  }

  foreach ($element['#attributes']['class'] as &amp;$class) {
    if ($class == 'is-active') {
      $class = 'active';
      break;
    }
  }

  $output = l($element['#title'], $element['#href'], $element['#localized_options']);

  return '&lt;li' . new Attribute($element['#attributes']) . '&gt;' . $output . $sub_menu . "&lt;/li&gt;\n";
}