hugo-book/assets/plugins/_numbered.scss

.book-page {
  .markdown {
    @for $h from 1 through 6 {
      > h#{$h} {
        counter-increment: h#{$h};
        counter-reset: h#{$h + 1};

        $content: "";
        @for $n from 1 through $h {
          $content: $content + 'counter(h#{$n})"."';
        }

        &::before {
          content: unquote($content) " ";
        }
      }
    }
  }

  + .book-toc nav ul {
    list-style: decimal;
  }
}