Do sins related to theft cease to exist after repenting from them and after the rights holders forgive what is owed, given that the debt is unknown and the rights holders forgive the remainder of the debt? And does Allah accept forgiveness for theft, or is it specific to other sins such as backbiting and slander?

You have done well by repenting of the great sin, and Allah forgives those who repent. You have fulfilled your duty by seeking absolution from those whose rights you violated, either by compensating them or by seeking their forgiveness for the remaining part, because absolution from injustices is achieved either by fulfillment or by discharge. The Prophet, peace and blessings be upon him, said: "Whoever has committed an injustice against his brother concerning his honor or anything else, let him seek absolution from him today before there will be neither a dinar nor a dirham."