Programmatically change the active theme in Drupal 7

gistfile1.aw

<?php

/**
 * Defines a theme callback function per registered path.
 */
function MODULENAME_menu_alter(&$items) {
	$items['node/%node']['theme callback'] = 'MODULENAME_default_node_theme';
	$items['node/%node/edit']['theme callback'] = 'MODULENAME_edit_node_theme';
	$items['node/%node/edit']['theme arguments'] = array(1);
}


/**
 * Theme name callback: without parameters.
 */
function MODULENAME_default_node_theme() {
	return 'garland';
}


/**
 * Theme name callback: with parameters.
 */
function MODULENAME_edit_node_theme($node) {
	return $node->type == 'page' ? 'seven' : MODULENAME_default_node_theme();
}


/**
 * Use hook_custom_theme() if the choice of theme doesn't depend on the path.
 */
function MODULENAME_custom_theme() {

	//Example: Changes the theme name depending if the user has a special role or not.
	global $user;
	if (in_array(variable_get('MODULENAME_special_role', 0), array_keys($user->roles))) {
		return 'bartik';
	}
}

?>

very nice