I know that reparation is required for sins against justice, but what about those sins (theft, detraction etc.) that were committed before baptism? Does baptism do away with the obligation to repair and make restitution for sins committed before it?
Yes, baptism remits all temporal punishment for sin.
Catechism of the Catholic Church:
1263 By Baptism all sins are forgiven, original sin and all personal sins, as well as all punishment for sin.