I want to change background-color or add text "It's draft" or anything in wp-admin/post.php according to the value of the post status. I know how to change it on the list page(wp-admin/edit.php). But I don't know what to do in wp-admin/post.php
Is there a dedicated filter / hook or html-class?
In your functions.php:
Use the admin_body_class hook to add the post status to the body of the post backend. Then create a custom stylesheet and enqueue it with the admin_enqueue_scripts hook.
Hope this helps.