Is it obligatory to return the value of stolen goods as of the day of the theft, or their current value, when the owner is unknown? And does Allah forgive the thief whose financial situation improved from the stolen money?

One must repent to Allah Almighty for theft by abandoning the sin, regretting it, and resolving never to return to it. Rights must be restored to their owners by returning the stolen item to them, or something similar if that is not possible, or its value on the day of the theft. Effort must be made to identify the owners, and if they cannot be reached, then its value should be given as charity on their behalf.