line-numbers.css

code.line-numbers {
  counter-reset: line_numbers calc(var(--line-numbers-init, 1) - 1);
}

code span.line-number {
  counter-increment: line_numbers;
  margin-right: 1em;
}

code span.line-number:before {
  content: counter(line_numbers, decimal-leading-zero);
  color: gray;
}

code span.highlight-line:before {
  color: yellow;
}