<style>
  div {
   color: red;
  }
</style>
<div id=om></div>

<script>
var stylesheet = document.styleSheets.item(0);
var rule = stylesheet.cssRules.item(0);
rule.insertRule("foo { }");
rule.insertRule("bar { }");
rule = stylesheet.cssRules.item(0);
rule.insertRule("baz { }");
var s = rule.style;
s.color = "green";
om.textContent = rule.cssText;
</script>