Ecco qui, grazie per l'aiuto:
<?php
/**
* @package WordPress
* @subpackage Default_Theme
*/
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" <?php language_attributes(); ?>>

<head profile="http://gmpg.org/xfn/11">
<meta http-equiv="Content-Type" content="<?php bloginfo('html_type'); ?>; charset=<?php bloginfo('charset'); ?>" />

<title><?php wp_title('&laquo;', true, 'right'); ?> <?php bloginfo('name'); ?></title>

<script type="text/javascript" language="JavaScript">
var now = new Date();
var hours = now.getHours();
var psj=0;
if (hours > 17 && hours < 6){
document.write(' <link rel="stylesheet" href="http://www.saviouz.netsons.org/wp-content/themes/default/style.css" type="text/css" media="screen">')
}
if (hours > 6 && hours < 17){
document.write(' <link rel="stylesheet" href="http://www.saviouz.netsons.org/wp-content/themes/default/style2.css" type="text/css" media="screen">')
}
</script>

<style type="text/css" media="screen">
<?php
// Checks to see whether it needs a sidebar or not
if ( empty($withcomments) && !is_single() ) {
?>
#page { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickbg-<?php bloginfo('text_direction'); ?>.jpg") repeat-y top; border: none; }
<?php } else { // No sidebar ?>
#page { background: url("<?php bloginfo('stylesheet_directory'); ?>/images/kubrickbgwide.jpg") repeat-y top; border: none; }
<?php } ?>

</style>

<?php if ( is_singular() ) wp_enqueue_script( 'comment-reply' ); ?>

<?php wp_head(); ?>


</head>
<body <?php body_class(); ?>>
<div id="page">


<div id="header" role="banner">
<div id="headerimg">
<h1><?php bloginfo('name'); ?></h1>
<div class="description"><?php bloginfo('description'); ?></div>
</div>
</div>
<hr />
Perché non dovrebbe funzionare lo script? Proprio non riesco a capire