It's hard to imagine someone who would really know 0 theory. Also it depends on what you call "good at chess". But the question I would ask myself is what would he play ? I think he would either play systems (which is already theory in a way), or the most natural moves.
Let's say you're white, I think you should try to cause him problems, so play e4.
He would probably either go for e5, or try to go for systems like the Pirc/modern or 1...b6. I wouldn't expect him to play Sicilian or Caro Kann, there are too many ways to get murdered in these openings if you don't know what you're doing. He might also play the french or maybe the Sandinavian with Qd8 for instance, thinking he's solid and trying to outplay you.
If he plays a system, that's a real problem because, he doesn't need to know much theory to understand the positions, and he probably has a lot of experience with them. I would suggest to play agressively but you need to find the right pawn break at the right moment, but he will be aware and he can play fast.
Against the rest, I think the main lines are often critical so if your opponent doesn't play precisely, you should get an edge and he might even go really wrong.
If you're black, again I would expect him either to play a system (english/Réti, London...) or to play 1. e4, followed by something like the Giocco piano. I think you should get relatively easy equality by play main lines, but it may be hard to really cause problems or even find constructive plans. So here I think agressive replies, even slightly unsound, might be the way to try to exploit his lack of opening prep. For instance, the Portuguese gambit in the Scandinavian. I used to play it and I think it could be refuted in several ways, but natural moves by white can be bad. Something like the Traxler could be interesting too if you get the chance to play it, but you're opponent would probably play d3 instead of Ng5. I'm also thinking about the Stafford gambit in the Petrov.
Anyway, the problem is you can't expect your opponents to know 0 theory so you can't create your repertoire on this assumption.
Let's say you're white, I think you should try to cause him problems, so play e4.
He would probably either go for e5, or try to go for systems like the Pirc/modern or 1...b6. I wouldn't expect him to play Sicilian or Caro Kann, there are too many ways to get murdered in these openings if you don't know what you're doing. He might also play the french or maybe the Sandinavian with Qd8 for instance, thinking he's solid and trying to outplay you.
If he plays a system, that's a real problem because, he doesn't need to know much theory to understand the positions, and he probably has a lot of experience with them. I would suggest to play agressively but you need to find the right pawn break at the right moment, but he will be aware and he can play fast.
Against the rest, I think the main lines are often critical so if your opponent doesn't play precisely, you should get an edge and he might even go really wrong.
If you're black, again I would expect him either to play a system (english/Réti, London...) or to play 1. e4, followed by something like the Giocco piano. I think you should get relatively easy equality by play main lines, but it may be hard to really cause problems or even find constructive plans. So here I think agressive replies, even slightly unsound, might be the way to try to exploit his lack of opening prep. For instance, the Portuguese gambit in the Scandinavian. I used to play it and I think it could be refuted in several ways, but natural moves by white can be bad. Something like the Traxler could be interesting too if you get the chance to play it, but you're opponent would probably play d3 instead of Ng5. I'm also thinking about the Stafford gambit in the Petrov.
Anyway, the problem is you can't expect your opponents to know 0 theory so you can't create your repertoire on this assumption.
