Add template classes to WordPress admin post editor

post-editor-classes.php

add_filter( 'admin_body_class', 'add_admin_body_template_class' );

function add_admin_body_template_class( $classes ) {

	if ( ! is_admin() ) {
		return $classes;
	}

	$queried_post_id = filter_input( INPUT_GET, 'post', FILTER_SANITIZE_NUMBER_INT );
	if ( ! $queried_post_id ) {
		return $classes;
	}

	$post = get_post( $queried_post_id );
	if ( ! $post || is_wp_error( $post ) ) {
		return $classes;
	}

	$template = get_page_template_slug( $post );
	if ( $template ) {
		$classes .= ' template-' . sanitize_title( $template );
	}

	if ( $post->ID === (int) get_option( 'page_on_front' ) ) {
		$classes .= ' is-front-page';
	}

	if ( $post->ID === (int) get_option( 'page_for_posts' ) ) {
		$classes .= ' is-blog-home';
	}

	if ( is_singular( $post ) ) {
		$classes .= ' post-type-' . sanitize_title( $post->post_type );
	}

	return $classes;
}