add-custom-body-class.php

<?php
// add custom body class for dontpaniclabs.com
add_filter( 'body_class', 'dpl_add_custom_body_class' );

function dpl_add_custom_body_class( $classes ) {
   $classes[] = 'dpl';
   return $classes;
}

// https://sridharkatakam.com/add-page-slug-body-class-static-pages-wordpress/
add_filter( 'body_class', 'dpl_body_class_for_pages' );
/**
 * Adds a css class to the body element
 *
 * @param  array $classes the current body classes
 * @return array $classes modified classes
 */
function dpl_body_class_for_pages( $classes ) {

  if ( is_singular( 'page' ) ) {
    global $post;
    $classes[] = 'dpl-page-' . $post->post_name;
  }
  return $classes;
}