If you know HTML , CSS, and Javascript you already know DHTML since that's what it is.
You need to learn DHTML DOM (Document Object Model) this is the API you use in javascript to effect page elements. The stuff you want will be in the style attributes. BEWARE though, since the style attributes and DOM itself are not fully supported in the browsers, in fact it's been best supported in IE, Netscape and Opera implementations of it were shotty or non-existent. Not sure if they have corrected this in both OPERA and Netscape.
//An example of javascript DOM
<script type="text/javascript">
header.style.color="red"
</script>
http://www.w3schools.com/dhtml/dhtml_dom.asp
