[WordPress] adding custom css classes to body or posts

svebal_custom_css_class.php

<?php
/** 
* to body of one page
* https://developer.wordpress.org/reference/functions/body_class/
**/
function svebal_custom_body_class_v1( $classes ) {
    if ( is_page( 'page-1' ) ) {
        $classes[] = 'own-class';
    }

    return $classes;
}
add_filter( 'body_class', 'svebal_custom_body_class_v1' );

/** to body of more pages **/
function svebal_custom_body_class_v2( $classes ) {
    if( is_page( array( 'page-1', 'page-2', 'page-2' ) ) ) {
        $classes[] = 'own-class';
    }
  
    return $classes;
}
add_filter( 'body_class', 'svebal_custom_body_class_v2' );

/** 
* to a post 
* https://developer.wordpress.org/reference/functions/post_class/    
**/
function svebal_custom_post_class( $classes ) {
    if( is_page( 'page-1' ) ) {
        $classes[] = 'own-class';
    }
    return $classes;
}
add_filter( 'post_class', 'svebal_custom_post_class' );
?>