Add 'inner' body class to all inner pages in WordPress.

functions.php

add_filter( 'body_class', 'sk_body_class' );
/**
 * Add "inner" class to 'body' element for inner pages
 * i.e., for all pages other than site's homepage/front page.
 *
 * @author Sridhar Katakam
 * @link   https://sridharkatakam.com/add-inner-body-class-inner-pages-wordpress/
 */
function sk_body_class( $classes ) {
	if ( ! is_front_page() ) {
		$classes[] = 'inner';
	}

	return $classes;
}