how to add css classes to body in WordPress

wp-body-class-function.php

<?php
/**
 * Generic function to add body classes
 * Can take either a string of space-separated classes or an array
 *
 * Requires PHP 5.3 for access to closures
 */

function es_add_body_class( $new_classes ) {
	// Turn the input into an array we can loop through
	if ( ! is_array( $new_classes ) )
		$new_classes = explode( ' ', $new_classes );

	// Add a filter on the fly
	add_filter( 'body_class', function( $classes ) use( $new_classes ) {
		foreach( $new_classes as $new_class )
			$classes[] = $new_class;

		return $classes;
	} );
}