You can use jQuery’s .content()
function to access it.
$('yourIframe').contents().find('#yourItemYouWantToChange').css({
opacity: 0,
color: 'purple'
});
Example
Here’s an example showing me applying css to the jQuery logo typically found in the top left of the screen. Mind you, it has to be same domain/ports etc, so that’s why my example features jsfiddle in the iframe.
http://jsfiddle.net/pPqGe/