Custom full width header image not showing up

I am building my own custom theme, for WordPress, and I have allowed for the customisation of a header background image. I can successfully change the header image, however when I do so, the header image appears nonexistent from the front end.

Functions.php code:

//home header
	$wp_customize->add_setting('home_img', array(
		'default' => get_template_directory_uri() .  '/images/web.jpg',
		'transport' => 'refresh',
	));
	$wp_customize->add_section('home_imgs', array(
		'title' => __('Home page banner image', 'Will Smith Media'),
		'priority' => 30,
	));
	$wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'header_image_control', array(
		'label' => __('Home Image', 'Will Smith Media'),
		'section' => 'home_imgs',
		'settings' => 'home_img',
	)));

functions.php customize css

function customize_css(){ ?>
	<style type="text/css">
		.site-header{
			background-image: <?php echo get_theme_mod('home_img'); ?>;
			width: 100%;
			height: 200px;
			display: block;
		}
	</style>

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