これは便利。ついでにlist-style-positionも…

ol.list_parentheses{
  padding:0 0 0 2em;
  margin:0;
}

ol.list_parentheses li{
  list-style-type:none;
  list-style-position:inside;
  counter-increment: cnt;
}
ol.list_parentheses li:before{
  display: marker;
  content: "(" counter(cnt) ") ";
}

詳しくはこちら