Adds line numbers to your pre blocks using css counters (enable lineanchors for pygments)

jekyll_pygments_line_numbers.css

pre {
    counter-reset: line-numbering;
    border: solid 1px #d9d9d9;
    border-radius: 0;
    background: #fff;
    padding: 0;
    line-height: 23px;
    margin-bottom: 30px;
    white-space: pre;
    overflow-x: auto;
    word-break: normal;
    word-wrap: normal;
}

pre a::before {
  content: counter(line-numbering);
  counter-increment: line-numbering;
  padding-right: 1em; /* space after numbers */
  width: 20px;
  text-align: right;
  opacity: 0.7;
  display: inline-block;
  color: #aaa;
  margin-right: 16px;
  border-right: 1px solid #ccc;
  padding: 2px 6px 2px 3px;
  font-size: 12px;
  -webkit-touch-callout: none;
  -webkit-user-select: none;
  -khtml-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

pre a:first-of-type::before {
  padding-top: 10px;
}

pre a:last-of-type::before {
  padding-bottom: 10px;
}

pre a:only-of-type::before {
  padding: 10px;
}