drupal/core/themes/bartik/css/components/book.css

.book-navigation .menu {
  border-top: 1px solid #d6d6d6;
}
.book-navigation .book-pager {
  border-bottom: 1px solid #d6d6d6;
  border-top: 1px solid #d6d6d6;
  margin: 0;
}