seperate_theme_wp.php

<?php
add_filter('template', 'change_theme');
add_filter('option_template', 'change_theme');
add_filter('option_stylesheet', 'change_theme');

function change_theme()
{
require_once("/path/to/Mobile_Detect.php");
$detect = new Mobile_Detect;
 
$deviceType = ($detect->isMobile() ? ($detect->isTablet() ? 'tablet' : 'phone') :
'computer');

switch($deviceType):

 case 'computer': $theme = 'default_theme_name';
                  break;

 case 'tablet':   $theme = 'tablet_theme_name';
                  break;

 case 'phone':    $theme = 'phone_theme_name';
                  break;

 default:         $theme = 'default_theme_name';
                  break;

 endswitch;

 return $theme;
}