Post-navigation-with-background-image.php

/**
 * Add featured image as background image to post navigation elements.
 *
 * @see wp_add_inline_style()
 */
function alpha_centauri_post_nav_background() {
	if ( ! is_single() ) {
		return;
	}

	$previous = ( is_attachment() ) ? get_post( get_post()->post_parent ) : get_adjacent_post( false, '', true );
	$next     = get_adjacent_post( false, '', false );
	$css      = '';

	if ( is_attachment() && 'attachment' == $previous->post_type ) {
		return;
	}

	if ( $previous &&  has_post_thumbnail( $previous->ID ) ) {
		$prevThumb = wp_get_attachment_image_src( get_post_thumbnail_id( $previous->ID ), 'alpha-centauri-navigation-img' );
		$css .= '
			.post-navigation .nav-previous { background-image: url(' . esc_url( $prevThumb[0] ) . '); }
		';
	}

	if ( $next && has_post_thumbnail( $next->ID ) ) {
		$nextThumb = wp_get_attachment_image_src( get_post_thumbnail_id( $next->ID ), 'alpha-centauri-navigation-img' );
		$css .= '
			.post-navigation .nav-next { background-image: url(' . esc_url( $nextThumb[0] ) . '); }
		';
	}

	wp_add_inline_style( 'alpha-centauri-style', $css );
}
add_action( 'wp_enqueue_scripts', 'alpha_centauri_post_nav_background' );

// Previous/next post navigation.
the_post_navigation( array(
	'next_text' => '<span class="meta-nav" aria-hidden="true">' . __( 'Next', 'alpha-centauri' ) . '</span> ' . '<span class="screen-reader-text">' . __( 'Next post:', 'alpha-centauri' ) . '</span> ' . '<span class="post-title">%title</span>',
	'prev_text' => '<span class="meta-nav" aria-hidden="true">' . __( 'Previous', 'alpha-centauri' ) . '</span> ' . '<span class="screen-reader-text">' . __( 'Previous post:', 'alpha-centauri' ) . '</span> ' . '<span class="post-title">%title</span>',
) );

Hello, I'm trying to implement this into my Twenty Sixteen child theme but can't get it work. Do you have any idea why can it be? I have added this function into functions.php of my child theme.