Figured it out:
Firstly there is a spelling mistake in that i’m using ‘post-box-image’ instead of ‘post_box_image’…
Secondly it’s better to do it this way in functions.php:
function jhwd_blog_theme_customize_css()
{
?>
<style type="text/css">
.post-block {
background-image:url('<?php echo get_theme_mod( 'post_box_image' );?>');
background-color: <?php echo get_theme_mod( 'post_box_color' );?>;
}
.post-block>h1, .post-boxes {
background-color: <?php echo get_theme_mod( 'post_boxes_items_color' );?>;
}
</style>
<?php
}
add_action( 'wp_head', 'jhwd_blog_theme_customize_css');