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

ee_taxonomy_body_class.php

<?php //Please do not add the opening PHP tag if you already have one

// add category slug to registration checkout and thank you 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( 'thank-you' ) && ! is_page( 'registration-checkout' ) ){
        return $classes;
    }
    
    //No transaction to begin with.
    $transaction = NULL;

    //If on the checkout page use the checkout SSN
    if ( is_page( 'registration-checkout') ) {
    	$checkout = EE_Registry::instance()->SSN->checkout();

    	if ( $checkout instanceof EE_Checkout ) {
        	$transaction = $checkout->transaction;
        }
    }

    //If on the thank you page use the query string and pull the transaction
    if ( is_page( 'thank-you' ) ) {
    	$transaction = EE_Registry::instance()->load_model( 'Transaction' )->get_transaction_from_reg_url_link();
    }
 	
 	//Check we have a transaction before we do anything else
    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;
}

ee_taxonomy_body_class_example.php

<?php //Please do not add the opening PHP tag if you already have one

// add category slug to registration checkout and thank you 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( 'thank-you' ) && ! is_page( 'registration-checkout' ) && ! is_page('registration-cancelled') ){
        return $classes;
    }
    
    //No transaction to begin with.
    $transaction = NULL;

    //If on the checkout page use the checkout SSN
    if ( is_page( 'registration-checkout') || is_page('registration-cancelled') ) {
    	$checkout = EE_Registry::instance()->SSN->checkout();

    	if ( $checkout instanceof EE_Checkout ) {
        	$transaction = $checkout->transaction;
        }
    }

    //If on the thank you page use the query string and pull the transaction
    if ( is_page( 'thank-you' ) ) {
    	$transaction = EE_Registry::instance()->load_model( 'Transaction' )->get_transaction_from_reg_url_link();
    }
 	
 	//Check we have a transaction before we do anything else
    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;
}