Detect Browser

functions.php

<?php 

add_filter('body_class','browser_body_class'); 
function browser_body_class($classes) { 
 
global $is_lynx, $is_gecko, $is_IE, $is_opera, $is_NS4, $is_safari, $is_chrome, $is_iphone; 
 
if($is_lynx) $classes[] = 'lynx'; 
 
elseif($is_gecko) $classes[] = 'gecko'; 
 
elseif($is_opera) $classes[] = 'opera'; 
 
elseif($is_NS4) $classes[] = 'ns4'; 
 
elseif($is_safari) $classes[] = 'safari'; 
 
elseif($is_chrome) $classes[] = 'chrome'; 
 
elseif($is_IE) $classes[] = 'ie'; 
 
else $classes[] = 'unknown'; 
 
if($is_iphone) $classes[] = 'iphone'; 
 
return $classes; 
} 

?>

If you want to use a different stylesheet for different browsers you can use HTML conditional tags or you can use this.