site/try/line_numbers.css

/* Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 * for details. All rights reserved. Use of this source code is governed by a
 * BSD-style license that can be found in the LICENSE file.
 */

.mainEditorPane {
  padding: 0px;
  counter-reset: dart-line-number;
}

.lineNumber {
  padding-left: 3.5em;
}

.lineNumber:before {
  counter-increment: dart-line-number;
  content: counter(dart-line-number) " ";
  position: absolute;
  left: 0px;
  width: 3em;
  text-align: right;
  background-color: lightgoldenrodyellow;
}