Theoretically, we would be able to colonize on Mars. No, we won't sustain off Mars for too long, but consider this: if we do plan to move to Mars, because Earth is about to die or whatever, we still can relay some resources off Earth. I'm sure if we ever reach that far where Earth is unlivable, we can build something cool that could transport resources, or make the ocean water drinkable, or something. After we deplete Earth's resources, our move to Mars will already be fully loaded with supplies we need. And if we are relaying resources, we would've already built a lot of crap that could help us live more comfortably. We would've either adapted or erased the toxic gas that surrounds Mars, built a bunch of stations and houses in one domed complex, drilled deep into Mars for water, and grown our own food.
In other answers I read, you guys are assuming we're dropping everything and going to Mars. The most reasonable answer is they're going to build a spaceships to send us there, send a few people to go first and build crap, or send robots to help build things for us. Although we have limited time on Earth if we do plan on colonizing on Mars, we have enough to make Mars a suitable stay for humans. Right now? Of course not. We may have had robots and things do things for us, but we have Trump as president; we aren't going to do good things for a long time. The only reason I would think people would want to move to Mars now is: they're edgy ("when Trump becomes president I'm flying to Mars"), they want to see if humans can survive (they can't right now), or they want to test what humans can do on Mars. But since you're talking theoretically, yes, I believe in the far future humans will be able to inhabit and live on Mars. How we effect Mars? Well, that's another question.