You are adding your styles with this
add_action('acf/input/admin_head', 'my_acf_admin_head');
which means that your function will run in the admin when acf does it’s admin_head stuff. But your targeting .main-content
, which is not a class that appears in the admin. I’m not sure where you’re trying to make this style appear.
Where exactly do you want to add this background?