gistfile1.php

<?php
/**
 * Uses a query string (color) to apply a body class.
 *
 * @param array $classes
 * @return array
 */
function ja_dynamic_body_class( $classes ){

	if ( empty( $_GET['color'] )
		return $classes;

	$classes[] = 'my-theme-' . esc_attr( $_GET['color'] );
 	
 	return $classes;
}
add_filter( 'body_class', 'ja_dynamic_body_class' );