Display featured image as header background

functions.php

add_action( 'wp_head', 'jk_header_background', 999 );

function jk_header_background() {
	global $post;

	$post_thumbnail_id  = get_post_thumbnail_id( $post->ID );
	$post_thumbnail_url = wp_get_attachment_url( $post_thumbnail_id );

	if ( $post_thumbnail_id ) {
		?>
		<style>
			// Replace '.my-header' with your header class.
			.my-header {
				background: url(<?php echo $post_thumbnail_url ?>);
				background-size: cover;
			}
		</style>
		<?php
	}
}