Add category name to body_class

functions.php

add_filter('body_class','add_category_to_single');
function add_category_to_single($classes) {
  if (!is_admin() && is_single() ) {
    global $post;
    foreach((get_the_category($post->ID)) as $category) {
      // add category slug to the $classes array
      $classes[] = $category->taxonomy . '-' . $category->slug;
    }
  }
  // return the $classes array
  return $classes;
}