Hi there,
it would be better to hook the CSS inline in the <head>
so:
Create a new Hook Element.
Add this to the Content:
<?php
$css = get_post_meta(get_the_ID(), 'css', true);
if (!empty($css)) { ?>
<style type="text/css">
<?php echo '.page-hero {background-color: ' . $css .';}'; ?>
</style>
<?php }
?>
Select the WP_Head
hook and check execute PHP then set your Display Rules to match the header element.
The above example has a Custom Field of css