Power Thief (any): steals the first power used on him. Can use a stolen power. Has to have a Power by the time the game ends, to win. Cannot absorb the factional Night Kill. Cannot steal a Power if he is dies the same Night. (i.e. if a Cop targets him and the Thief dies the same Night, the Cop keeps his Power)
Microfish_1: so... can the thief flip alignment mid-game if he is targeted by a member of the other team? (IE if a scum rolecop targets a town thief, does the thief become scum? or does scum lose the rolecop? If the scum thief is targeted by a town cop, does he become the scum variant of that cop?)
The thief just steals powers, not alignment. So yes, if a scum role-cop targets the thief, scum lose a PR. The former thief becomes a role-cop, keeping whatever alignment he has, while the former role-cop becomes a goon.