I am not a hugo expert, but i think the issues you are seeing are connected to a certain variable that hugo uses called “baseurl” - you might have to tweak things a bit for the right resources to be found - at the moment, your paths to your css clearly aren’t resolving on those sub pages.
If you search through the forums for “baseurl” you will see lots of similar posts as to this, and this comment from marcus might be interesting:
