Assuming that A is telling the truth, then if B lies, it may not be stolen by A, if C lies, it may be stolen by C, and if D lies, it may not be stolen by B. So we can know that C is a thief.
Assuming that B is telling the truth, then the lies told by the other three people, A can steal, C can steal and D can't steal B, then we can know that A and C are thieves, which is impossible.
So answer C is a thief.