Are you able to use PHP on your server?
If so, you could solve the problem through a little PHP command.
PHP Code:
<?php
$css = rand(0, 2);
if($css === 0){
echo '<link rel="stylesheet" type="text/css" href="/sylesheet1.css" />';
}
elseif($css === 1){
echo '<link rel="stylesheet" type="text/css" href="/sylesheet2.css" />';
}
else{
echo '<link rel="stylesheet" type="text/css" href="/sylesheet1.css" />';
}
?>
Replace the 'href' bit with a link to whatever stylesheet you are using and place the above code inside the <head> tags of your website.
I hope that works, it seemed to when I tested it.
PS Sorry if that shouldn't have been posted in this forum.