Better:
<?php
session_start();
$themes = array(
'red' => 'Red',
'greenish' => 'Green/Blue',
'transparent' => 'Glass',
'fish' => 'Sea Life',
'grass' => 'Natural'
);
$defaultTheme = @$_POST['theme'];
if (!isset($themes[$defaultTheme]))
$defaultTheme = key($themes);
$_SESSION['preferred_theme'] = $defaultTheme;
?>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<link rel="stylesheet"
type="text/css"
href="<?php echo $defaultTheme; ?>">
<?php foreach ($themes as $file => $title): ?>
<link rel="alternate stylesheet"
type="text/css"
href="<?php echo $file; ?>.css"
title="<?php echo $title; ?>">
<?php endforeach; ?>
</head>
<body>
<form action="<?php echo $_SERVER['REQUEST_URI']; ?>">
<div>
<select name="theme">
<?php foreach ($themes as $file => $title): ?>
<option value="<?php echo $file; ?>">
<?php echo $title; ?>
</option>
<?php endforeach; ?>
</select>
<input type="submit" value="Select">
</div>
</form>
</body>
</html>Schmoopy, watch that pseudo-XHTML.