<?php
// set the url of the page you would have previously linked to in the iframe
//$url = 'http://www.website-to-request.com/';
$url = 'http://www.example.com/';
// Setup the new css you want to inject into the page
$css = '
<style type="text/css">
body { background: green; }
.some-other-elements { ... }
</style>
';
// Get the file contents (you may want to replace this with a curl request
$site_content = file_get_contents($url);
// a simple way to inject style into this page would be to ad it directly above the closing head tag (if there is one)
// this can be changed to any element, or even using the dom class you could ammend this with more detail.
$site_content = str_replace('</head>', $css.'</head>', $site_content);
// you may also need to inject a base href tag so all the links inside are still correct
// comment out the next line if not needed
$site_content = str_replace('<head>', '<head><base href="'.$url.'" />', $site_content);
// return the site contents to the browser
echo $site_content;
?>