mu-theme-switcher.php

<?php
/*
Plugin Name: Mobile Switcher
Version: 0.1
Author: Ben May

To toggle, use the [show_theme_switch_link] shortcode. 

*/
class mysiteMobile {

    const key       = 'my_cookie_to_set_desktop';
    const theme     = 'site-main-theme';
    const style     = 'site-mobile-theme';

    private $shown = 'desktop'; // Always assumed
    private $hasCookie;

    public function __construct()
    {
        $this->hasCookie = ( isset( $_COOKIE[ self::key ] ) );

        // If it's a mobile device, and there is no cookie, show mobile.
        if( mysite_is_mobile() && ( !$this->hasCookie ) )
        {
            add_filter( 'stylesheet', array( $this, 'swap_style') );
            add_filter( 'template', array( $this, 'swap_theme') );
            $this->shown = 'mobile';
        }

        add_action( 'init', array( $this, 'redirect_header_check' ) );
        add_shortcode( 'show_theme_switch_link', array( $this, 'footer_link' ) );
    }
    
    public function redirect_header_check()
    {
        if( isset($_GET[ self::key ] ) ):
                
            if( $_GET[ self::key ] == 'desktop' ):
                // Set cookie to force desktop for 14 days.
                setcookie( self::key, true, time()+60*60*24*14);
            else:
              setcookie( self::key, '', current_time( 'mysql' ) - 3600 );
            endif;
            
            wp_redirect( remove_query_arg( self::key ) );
            exit;
            
        endif;
    }

    public function footer_link()
    {
        if ( $this->shown == 'mobile')
        {
            $return = '<a rel="external" data-ajax="false" href="?'.self::key.'=desktop">Switch to Desktop Version</a>';
        }
        elseif($this->hasCookie)
        {
            $return = '<a href="?'.self::key.'=mobile">Switch to Mobile Version</a>';
        }
        return $return;
    }

    public function swap_theme()
    {
        return self::theme;
    }
    public function swap_style()
    {
        return self::style;
    }
}

// requires mysite_is_mobile function in wp-config
if( function_exists( 'mysite_is_mobile' ) )
    $roarMobile = new mysiteMobile();