add_pager: Override pager

To change the pager, override the add_pager() method.

/**
 * Adds a pager to the form.
 *
 * @param $form
 *   The form array to add the pager to.
 * @param $form_state
 *   The current state of the form.
 * @return
 *   The original form with the pager added.
 */
public function add_pager($form, $num_results) {

  // Determine the current page and offset using the query parameters.
  $q = drupal_get_query_parameters();
  $offset = (isset($q['offset'])) ? $q['offset'] : 0;
  $page_num = (isset($q['page_num'])) ? $q['page_num'] : 1;

  // Do your logic to render your pager.
  $output = '';

  // Add the pager to the result page.
  $form['pager'] = [
    '#type' => 'markup',
    '#markup' => $output,
    '#prefix' => '<div class="pager-container"><div class="pager">',
    '#suffix' => '</div></div>',
    '#weight' => 100,
  ];

  return $form;
}