parse static method
Implementation
static CSSBorder? parse(String? value) {
  if (value == null) return null;
  var match = pattern.firstMatch(value);
  if (match == null) return null;
  var sizeStr = match.group(1);
  var styleStr = match.group(2);
  var colorStr = match.group(3);
  var size = CSSLength.parse(sizeStr);
  var style = parseCSSBorderStyle(styleStr);
  var color = CSSColor.parse(colorStr);
  return CSSBorder(size, style, color);
}