Changing theme based on http headers
A simple way to alter your theme based on HTTP headers can be accomplished using the following code:
<?php
$headers = drupal_set_header();
if (strstr($headers, 'HTTP/1.0 404 Not Found')) {
// do something, output something, change template file, etc...
}
?>Works great and works with all other headers. Great for having a completely different layout/theme for 404 pages.
Actual Change of a Theme
If you need to actually change a theme, you need to write the following in a module's init hook:
function yourmodule_init() {
global $custom_theme, $theme;
//... some logic ...
$theme = null;
$custom_theme = 'chameleon'; // 'chameleon' being the theme you want to set
}