How to apply css to an iframe

There are two different things here: the style of the iframe block and the style of the page embedded in the iframe. You can set the style of the iframe block the usual way:
<iframe name='iframe1' id="iframe1" src="empty.htm" frameborder="0" border="0" cellspacing="0" style="border-style: none;width: 100%; height: 120px;"></iframe>
The style of the page embedded in the iframe must be either set by including it in the child page:
<link type="text/css" rel="Stylesheet" href="Style/simple.css" />
Or it can be loaded from the parent page with Javascript:
var cssLink = document.createElement("link") cssLink.href = "style.css"; cssLink .rel = "stylesheet"; cssLink .type = "text/css"; frames['frame1'].document.body.appendChild(cssLink);