Add event's category slug to registration checkout page's body class

ee_taxonomy_body_class.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file
// add category slug to registration checkout page's body class

add_filter( 'body_class', 'jf_ee_return_event_tax_term_on_spco' );

function jf_ee_return_event_tax_term_on_spco( $classes ){
    // get out if this isn't the reg checkout page
    if (! is_page( 'registration-checkout' ) ){
        return $classes;
    }

    $events = array();
    $checkout = EE_Registry::instance()->SSN->checkout();
    if ( $checkout instanceof EE_Checkout ) {
        $transaction = $checkout->transaction;
        if ( $transaction instanceof EE_Transaction ) {
            foreach ( $transaction->registrations() as $registration ) {
                if ( $registration instanceof EE_Registration ) {
                    $event = $registration->event();
                    if ( $event instanceof EE_Event ) {
                        $events[ $event->ID() ] = $event;
                        $category = $event->first_event_category();
                        if ( $category instanceof EE_Term ) {
                            $category_slug = $category->slug();
                            $classes[] = $category_slug;
                        }
                    }
                }
            }   
        }
    }
    return $classes;
}