If the styling is in HTML then it's inline styles. There's no CSS Wizardry that will override it. What you need to do is directly modify the inline styles.
Here's what you're looking for:
var elem = document.getElementById("elementID");
elem.setAttribute("style", "font-size: 2em");
You'll have problems in IE with setAttribute
so if that's a concern ... you'll need to write a fallback to handle it.