Then it won't be theft. Theft by definition is illegal.
The problem you and Fred have is that you both equate solitary individuals with states. An individual is one person in a society and if he wants something, he has to persuade the society its a good idea. A State is the embodiment of a society; all your rights and freedoms are provided by the state. Therefore its bad logic to try to redefine "tax" to be illegal and "theft" to be legal. Its the law of the State that provides the meaning of both words.
I have a neighbor girl who has 50 pieces of gold. I want it. However, I don't want to steal it. I go to my local politician (a legally elected representative) and tell him I want the girl's gold. He passes a law permitting me to abscond with it. It is no longer theft. I take the 50 pieces.
Amazing how my conscience is assuaged. Very interesting concept.
P.S. I also have an idea for an assassination... You wouldn't mind helping me out with that would you?