different-theme.php

<?php
/*
Plugin Name: Different theme
Description: Use a different theme on certain conditions
Author: Nikolay Bachiyski
Version: 1.0
*/

class Different_Theme {
	function __construct() {
		add_filter( 'stylesheet', array( &$this, 'theme_stylesheet' ) );
		add_filter( 'option_stylesheet', array( &$this, 'theme_stylesheet' ) );
		add_filter( 'template', array( &$this, 'theme_template' ) );
		add_filter( 'option_template', array( &$this, 'theme_template' ) );
	}

	/**
	 * Decides whether the use a different theme from the current one.
	 * 
	 * @return string|bool Returns a the relative path to the themes directory if a different
	 * 		theme should be used or false if the theme should not be changed
	 */
	function change_theme_to( $current_theme ) {
		/*
		Example:
			if ( isset( $_SERVER['HTTP_USER_AGENT'] ) && strpos( $_SERVER['HTTP_USER_AGENT'], 'iPhone' ) !== false ) {
				return 'iphone-theme';
			}
		*/
		return $current_theme;
	}
	
	function theme_template( $current_theme_template ) {
		$new_template = $this->change_theme_to( $current_theme_template );
		if ( $new_template != $current_theme_template ) {
			// use parent template in case of a child theme
			$theme_data = get_theme_data( get_theme_root(). "/$new_template/style.css" );
			if ( $theme_data['Template'] ) {
				$new_template = $theme_data['Template'];
			}
		}
		return $new_template;
	}
	
	function theme_stylesheet( $current_theme_stylesheet ) {
		return $this->change_theme_to( $current_theme_stylesheet );
	}	
}

$wp_plugin_different_theme = new Different_Theme;