滤器pre_option_stylesheet. 未测试的示例代码：

add_filter( \'pre_option_stylesheet\', \'wpse_100854_switch_stylesheet\' );

function wpse_100854_switch_stylesheet( $false )
{
    if ( empty ( $_GET[\'theme\'] )
        return $false;

    $themes = wp_get_themes( array( \'allowed\' => true ) );

    if ( isset ( $themes[ $_GET[\'theme\'] ] ) )
        return $_GET[\'theme\'];

    return $false;
}

要仅更改特定页面的模板，请连接到template_include:

add_filter( \'template_include\', \'wpse_100854_switch_template\' );

function wpse_100854_switch_template( $template )
{
    if ( empty ( $_GET[\'template\'] )
            return $false;

    // do not allow to break out of the directory
    $new  = ltrim( $_GET[\'template\'], \'.\' ); 
    $path = get_template_directory() . "/templates/$new.php";

    if ( file_exists( $path ) )
        return $path;

    return $template;
}