Change Color With CSS Class Instead Of Html Font Tags

I'm using javascript to change the color of certain words in an html document. Right now it uses <font> to make the change. I would like to use a CSS class named "alert" from an external style sheet to make the change. I've tried multiple things but no luck (className="alert", setClassName="alert" )
What is the correct way to replace the font part with a CSS class? Here is the working script:

<script type = "text/javascript">
window.onload = function (){
var text = document.getElementsByTagName('body')[0].innerHTML;
text = text.replace(/disabled/gi, "<font color=red>Disabled</font>" );
text = text.replace(/locked/gi, "<font color=red>Locked</font>" );
document.getElementsByTagName('body')[0].innerHTML = text;
}
</script>