tim.kicker.dev/css/prism-line-numbers.css

pre[class*="language-"].line-numbers {
  position: relative;
  padding-left: 3em;
  counter-reset: linenumber;
}

pre[class*="language-"].line-numbers > code {
  position: relative;
  white-space: inherit;
}

.line-numbers .line-numbers-rows {
  position: absolute;
  pointer-events: none;
  top: 0;
  font-size: 100%;
  left: -3.8em;
  width: 3em; /* works for line-numbers below 1000 lines */
  letter-spacing: -1px;
  border-right: 1px solid #999;

  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

.line-numbers-rows > span {
  display: block;
  counter-increment: linenumber;
}

.line-numbers-rows > span:before {
  content: counter(linenumber);
  color: #999;
  display: block;
  padding-right: 0.8em;
  text-align: right;
}