There are many theories about colonizing Mars, from practical, short-term ideas, like bio-habbitats we ship there, all the way to the idea of actually drilling into the polar caps to create a viable atmosphere (I also vaguely remember a theory about using some kind of fungus to make the atmosphere).h
So far, the widely accepted theory is that Mars can be colonized and even support an atmosphere (at least as of a year ago when I learned that). So, if that is the case, and we are willing to wait for the atmosphere to form, we could, in theory, call Mars a second Earth, in regards to living there.
I fully believe we are capable of colonizing other worlds, from forming atmospheres, to shooting “tuna-can-houses” to the surface. It’s just a matter of how much time and how much money we are willing to expend… For now.