Developer 

Hook: change the classes 

Before the body tag is rendered, the hook pageclassesCssHook gets all collected classes and may add, change or remove them. The array is passed by reference, keys and values are the (HTML-escaped) class names.

Register the hook in the ext_localconf.php of your extension:

$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['pageclasses']['pageclassesCssHook'][]
    = \MyVendor\MyExtension\Hook\PageclassesHook::class;

namespace MyVendor\MyExtension\Hook;

class PageclassesHook
{
    /**
     * @param object $parentObject the plugin \SvenT\Pageclasses\Plugin\Pageclasses
     * @param array<string, string> $pageClasses
     */
    public function processPageclassesCssHook($parentObject, array &$pageClasses): void
    {
        $pageClasses['is-christmas'] = 'is-christmas';
    }
}

TypoScript 

The body tag is rendered by the USER object plugin.tx_pageclasses_pi1 (class \SvenT\Pageclasses\Plugin\Pageclasses , method main). It returns the complete body tag, e.g. <body class="home layout_0">, or <body> if there is no class.