Difference between modified post and new post

Add a post class when the post modified time doesn’t match publish time. Then style it with that class.

add_filter( 'post_class', 'wpse244651_add_modified_class' );


function wpse244651_add_modified_class( $classes ) {
    global $post;
    if( get_the_modified_time( $post->ID ) != get_the_time( $post->ID ) ) {
        $classes[] = 'modified-post';
    }
    return $classes;
}