book-2.0.x-dev/modules/book_olivero/book_olivero.module

<?php

/**
 * @file
 * Tell Drupal about templates.
 */

/**
 * Implements hook_theme().
 */
function book_olivero_theme(): array {
  return [
    'book_navigation' => [
      'template' => 'book-navigation',
    ],
    'book_tree' => [
      'template' => 'book-tree',
    ],
  ];
}