<?php if ( is_category('15') ) {
  <link rel="stylesheet" href="<?php bloginfo('template_url'); ?>/cat-15.css" 
type="text/css" media="screen" />;
<?php } else { ?>
   <link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); ?>" 
type="text/css" media="screen" />
<?php } ?>

<?php
  $post = $wp_query- >post;
 if ( in_category('3') ) {
 include(TEMPLATEPATH . '/cat3.php’);
 } elseif ( in_category('4') ) {
 include(TEMPLATEPATH . '/cat4.php');
 } else {
 include(TEMPLATEPATH . '/cat.php');
 } ? >