functions.php

<?php

add_action( 'wp_enqueue_scripts', 'mh_twentysixteen_child_style' , 5 );

function mh_twentysixteen_child_style() {
    wp_enqueue_style( 'twentysixteen-parent-style', get_template_directory_uri() . '/style.css' );
}

function twentysixteen_post_thumbnail() {
    if ( post_password_required() || is_attachment() || ! has_post_thumbnail() ) {
        return;
    }

    if ( is_singular() ) :
    ?>

    <div class="post-thumbnail">
        <?php the_post_thumbnail('full'); ?>
    </div><!-- .post-thumbnail -->

    <?php else : ?>

    <a class="post-thumbnail" href="<?php the_permalink(); ?>" aria-hidden="true">
        <?php the_post_thumbnail( 'full', array( 'alt' => the_title_attribute( 'echo=0' ) ) ); ?>
    </a>

    <?php endif; // End is_singular()
}