FB: ACF field to body_class() via filter

functions.php

<?php

/**
 * WP Filter which adds acf field value to body classes for all pages but home.
 * 
 * @see https://codex.wordpress.org/Plugin_API/Filter_Reference/body_class
 */
function THEME_NAME_body_class( $classes ) {
  if ( function_exists( 'get_field' ) && ! is_home() ) {
    $classes[] = get_field( 'page_background' );
  }
  return $classes;
}
add_filter( 'body_class', 'THEME_NAME_body_class' );