I would be less worried about the thief's car and more worried about the car the thief runs into.
You can sue anyone for anything. That would be a very uphill lawsuit in normal circumstances (the thief is being negligent by trying to open a package in a moving vehicle and any harm is a result of that negligence). When you add in the fact that the package is stolen it's highly unlikely to succeed. However, I write this from a US perspective. If self defense case law is any indication I suspect that the thief may have a case in the UK.
At the end you would just end up going to the police confess the crime, open the door to be prosecuted for other crimes. And yet end up proving nothing about the other person from whom you stole.
It would be shooting in the foot at epic proportions.
So you'd have to admit to a felony to sue or press charges against box maker. Second when committing a felony, in many jurisdiction the felon is automatically on the hook for any further 'bad things' that happen during the crime up to and including deaths. So if the felonious package thief got in an accident while opening a stolen package, it could all fall on them as a matter of statute in many places.
There is nothing about his project designed to cause any bodily injury. This is a very important detail.