Add category name (slug) as CSS body class in WordPress

add_filter('body_class','mrfx_add_category_to_single');
function mrfx_add_category_to_single($classes, $class) {
	if (is_single() ) {
		global $post;
		foreach((get_the_category($post->ID)) as $category) {
			// add category slug to the $classes array
			$classes[] = $category->category_nicename;
		}
	}
	// return the $classes array
	return $classes;
}