How to add Page slug as a body class for static Pages in WordPress. https://sridharkatakam.com/add-page-slug-body-class-static-pages-wordpress/

functions.php

add_filter( 'body_class', 'sk_body_class_for_pages' );
/**
 * Adds a css class to the body element
 *
 * @param  array $classes the current body classes
 * @return array $classes modified classes
 */
function sk_body_class_for_pages( $classes ) {

	if ( is_singular( 'page' ) ) {
		global $post;
		$classes[] = 'page-' . $post->post_name;
	}

	return $classes;

}