devdocs/assets/stylesheets/pages/_d3.scss

._d3 {
  > h2 { @extend %block-heading; }
  > h3 { @extend %block-label; }
  > h4 { font-size: 1rem; }
  > h6 { @extend %block-label, %label-blue; }

  > h6 > .source {
    float: right;
    font-weight: normal;
  }

  code { @extend %label; }
  blockquote { @extend %note, %note-blue; }
}