First, please validate your PRO theme.
Regarding the styling issue, please do add Page Not Found custom CSS on X > Launch > Theme Options > Global CSS instead. We need to add those CSS on Global CSS so it can be available site wide including 404 page like this.
I can see you have two header.
Unfortunately, as of the moment, a specific header should be set as global for it to work on 404 page. Current headers is not assigned as global. We have already added a request to add 404 page on the assignment area of the header builder. We will let you know once we have an update.