当 split_by 未使用时,在书本中强调章节标题编号(仅)

underline chapter title number (only) in bookdown when split_by not used

如何在章节标题的数字下添加3px的线,如下所示: https://bookdown.org/yihui/bookdown/introduction.html 不强调第 (1.1) 节。出现问题时 split_by: section 未在 _output.yml 中使用。 (我不想分节)

#_output.yml
bookdown::gitbook:
  css: style.css
  split_by: section #<- NOT HAVING THIS

#css
.level1 .header-section-number {
  display: inline-block;
  border-bottom: 3px solid;
}

我设法做到了:

.level1 .header-section-number {
  display: inline-block;
  border-bottom: 3px solid;
}

.level2 .header-section-number {
  border-bottom: none;
}