Technically, it's impossible to hack a cold wallet. However, this doesn't mean hardware wallets are "theft-proof." Indeed, a few studies suggest hackers could decode a hardware device's PIN when it's plugged into a computer.
While someone could steal funds from a cold wallet, it's less likely versus a hot wallet. If you keep your cold wallet's PIN and private key in a fireproof safe, the chances you'll lose your crypto are low.