devdocs/assets/stylesheets/pages/_bootstrap.scss

._bootstrap {
  @extend %simple;

  h4 > code, h5 > code, strong > code { @extend %label; }

  h2 > small {
    color: var(--textColorLight);
    float: right;
  }

  .bs-callout { @extend %note; }
  .bs-callout-info { @extend %note-blue; }
  .bs-callout-danger { @extend %note-red; }
  .bs-callout > h4, .bs-callout > h5 { margin-top: .25rem; }

  .text-danger { @extend %label, %label-red; }

  p.bs-example {
    padding: .375rem .625rem;
    line-height: 1.5;
    @extend %heading-box;
  }

  div.bs-example {
    @extend %pre-heading;
  }

  a.thumbnail {
    display: block;
    padding: .25em;
    @extend %box;

    &:after { content: none; }
    + h4 { margin: .75em 0 .5em; }
    > img { display: block; }
  }

  .col { margin-bottom: 1.5em; }

  @media (min-width: 800px) {
    .row {
      display: flex;
      flex-wrap: wrap;
    }

    .col {
      flex: 0 1 auto;
      padding: 0 1em;
      width: 33.33%;
      -moz-box-sizing: border-box;
           box-sizing: border-box;
    }
  }
}