You'll have to use Cookies or Sessions and then use that
code:
<?php
$myStyleColour = $_POST['styleColour'];
if($myStyleColour=="red"){
print '<link rel="stylesheet" type="text/css" href="css/styles2.css" />';
} else{
/*if($myStyleColour=="normal")*/
print '<link rel="stylesheet" type="text/css" href="css/styles.css" />';
}
?>
on the pages where you want to switch the styles, but you'll also need another script to actually do the switching and set the Cookie or session accordingly