Can a 51% Attack Be Used to Steal Coins That the Attacker Does Not Already Possess?

No, a 51% attack cannot be used to create new coins out of thin air or steal coins from other users' wallets that the attacker does not already control. The attack's power is limited to manipulating the transaction history and the order of transactions.

The primary goal is a double-spend, which involves reversing a transaction of the attacker's own coins after they have been spent. It does not grant the ability to forge cryptographic signatures or bypass the fundamental rules of the protocol regarding coin ownership.