[WordPress][functions] Add body classes for active menu locations and active widget areas. In reply to this post (🇩🇪): http://torstenlandsiedel.de/2016/05/01/css-klasse-hinzufuegen-fuer-mobilgeraete/

function-body-class-active-menus.php

<?php
/**
 * Alters body class for populated menu locations.
 * @param  array $classes Registered body classes
 * @return array          Altered body classes
 */
function ch20160502__body_class_active_menus( $classes ) {

	$menu_locations = get_nav_menu_locations();

	if ( empty( $menu_locations ) )
	 	return $classes;

	foreach ( $menu_locations as $location => $items_count ) {
		if ( $items_count > 0 )
			$classes[] = sprintf( 'menu-%s-active', esc_attr( $location ) );
	}

	return $classes;
}
add_filter( 'body_class', 'ch20160502__body_class_active_menus' );

function-body-class-active-sidebars.php

<?php
/**
 * Alters body class for active sidebars.
 * @param  array $classes Registered body classes
 * @return array          Altered body classes
 */
function ch20160502__body_class_active_sidebars( $classes ) {

	foreach ( $GLOBALS['wp_registered_sidebars'] as $id => $args ) {
		if ( is_active_sidebar( $id ) )
			$classes[] = sprintf( 'widget-area-%s-active', esc_attr( $id ) );
	}

	return $classes;
}
add_filter( 'body_class', 'ch20160502__body_class_active_sidebars' );