Drupal (6 & 7): programmatically change the active theme

// DRUPAL 6
function mymodule_init(){
	global $custom_theme;
	$custom_theme = 'garland';
}
 
 
 
// DRUPAL 7
/**
 * Defines a theme callback function per registered path.
 */
function mymodule_menu_alter(&$items) {
	$items['node/%node']['theme callback'] = 'mymodule_default_node_theme';
 
	$items['node/%node/edit']['theme callback'] = 'mymodule_edit_node_theme';
	$items['node/%node/edit']['theme arguments'] = array(1);
}
 
 
/**
 * Theme name callback: without parameters.
 */
function mymodule_default_node_theme() {
	return 'garland';
}
 
 
/**
 * Theme name callback: with parameters.
 */
function mymodule_edit_node_theme($node) {
	return $node->type == 'page' ? 'seven' : mymodule_default_node_theme();
}
 
 
/**
 * Use hook_custom_theme() if the choice of theme doesn't depend on the path.
 */
function mymodule_custom_theme() {
 
	//Example: Changes the theme name depending if the user has a special role or not.
	global $user;
	if (in_array(variable_get('mymodule_special_role', 0), array_keys($user->roles))) {
		return 'bartik';
	}
}