I was studying something which quoted the Talmud as saying that all seven Noahide laws are capital crimes. What do I make of this? Does Judaism say that a non-Jew is supposed to be executed for any crime, including, for example, theft?

The way I have always understood the issue is that the death penalty is granted to them as an option: Were it not for Divine license, the State would have no intrinsic right to impose it unilaterally. Another one of the Seven Laws is the deployment of a just legal and judicial system. My understanding is that they need to have a just system, but then may decide on their own which penalties are warranted for which crimes.