If you need to style an element using Javascript then you can use the style object to support all your CSS needs.
<html>
<body>
<p id="p1">Hello World!</p>
<script>
document.getElementById("p1").style.color = "red";
</script>
<p>The paragraph above was changed by a script.</p>
</body>
</html>
If you need to do this purely in a Javascript file itself, then:
// Get a reference to the element
var myElement = document.querySelector("#p1");
// Now you can update the CSS attributes
myElement.style.color = "red";
myElement.style.backgroundColor = "yellow";
Note that where CSS properties would use dashes to separate words, like background-color, Javascript instead uses camel casing and drops the space.
Example: background-color would become backgroundColor.