首先，我要说，我认为这不是最好的想法，但为了回答这个问题。。。

总体思路是过滤template, option_template, 和option_stylesheet 并返回所需的模板段塞。其余的代码只是设置和读取cookie。

<?php
/**
 * Plugin Name: WPD Theme Switcher
 */

class WPD_Theme_Switcher {

    private $themes = array(
        \'twentythirteen\',
        \'twentyfourteen\',
        \'twentyfifteen\'
    );
    private $current_theme = \'\';
    private $cookie = \'wpd_theme_switcher_cookie\';

    function __construct() {

        if( empty( $this->current_theme ) && !isset( $_COOKIE[ $this->cookie ] ) ) {
            $this->current_theme = $this->themes[ array_rand( $this->themes ) ];
            setcookie( $this->cookie, $this->current_theme, time() + (10 * 365 * 24 * 60 * 60) );
        } else {
            $this->current_theme = $_COOKIE[ $this->cookie ];
        }

        // don\'t switch themes for admin requests
        if( ! is_admin() ){
            add_filter( \'template\', array( $this, \'theme_switcher\' ) );
            add_filter( \'option_template\', array( $this, \'theme_switcher\' ) );
            add_filter( \'option_stylesheet\', array( $this, \'theme_switcher\' ) );
        }

    }

    function theme_switcher(){
        return $this->current_theme;
    }

}
new WPD_Theme_Switcher();