How can stolen money be returned and repentance made after the father's death, knowing that the father died before the money was returned and it is unknown whether he was pleased or not?

You are obligated to repent to Allah Almighty for taking your father's money unjustly. This involves abstaining from the sin, regretting it, and resolving not to return to it, along with restoring the right to your deceased father's heirs according to their prescribed shares. You should also frequently supplicate and seek forgiveness for your father, and ask Allah to forgive you.