Many definitions of "theft" include specific text regarding "the rightful owner" or a variant necessitating that the victim of theft owned the property. (emphasis mine)
For example, The Free Dictionary defines theft as:
A criminal act in which property belonging to another is taken without that person's consent.
Merriam Webster defines it similarly:
the act of stealing; specifically : the felonious taking and removing of personal property with intent to deprive the rightful owner of it
How do these definitions apply if the person being stolen from is not actually the rightful owner, and has in fact stolen the property himself? What criminal repercussions do thieves of stolen property face?