While it's true that Theravada generally focuses on the individual aspects of kamma since the vast majority of the teachings in the Pali Canon deal with actions on an individual level (e.g., AN 5.57, MN 61, MN 136, etc.), there is one section at the beginning of DN 16, dealing with the "the growth of the Vajjis," that seems to allude to a type of collective kamma at a 'national' level, i.e., the Buddha asks Ananda if the Vajjis as whole do certain things (which are arguably skillful actions), and after Ananda answers yes, the Buddha states that, "So long, Ananda, as this is the case, the growth of the Vajjis is to be expected, not their decline." The Buddha doesn't call it collective kamma, of course, but I think the general principle is there.
That said, the subject of collective kamma bothers me for a number of reasons. It's not that I think it's impossible for there to be a concentration of individual kammic results in one place, due to the combined actions of a cohesive whole, such as from the citizens of a country. I think it's a valid frame of reference and can help motive people to be more socially engaged in their community/society. But even so, I still tend to view them as individual actions with individual results that are, or least appear to be, similar (i.e., nations don't have intentions, the citizens of those nation do). Furthermore, the idea of collective kamma bothers me when people use it unskillfully, e.g., when people harbour intense guilt for actions that they themselves didn't commit, and then end up creating more suffering for themselves by cultivating unwholesome mental states in the belief that they're somehow responsible, perhaps out of a desire to be punished or something.
I think that if people use the idea of collective kamma in a skillful way, such as being more active in their country's politics to help steer public policy, giving humanitarian aide, etc., it can be a good thing that helps to lessen suffering all around. But given the tendency of people to harbour guilt in numerous and often unwholesome ways, I feel that the idea of collective kamma can do more harm than good, such as leading to taking on the 'weight of the world' when what we really want to do is cast our burdens aside. I think if we view the teachings on kamma as teachings about personal responsibility, that our actions not only affect ourselves but those around us, then we're on the right track. When it comes to carrying the guilt of other's misdeeds, however, I think that's a self-imposed burden that we needn't bear. We can't change the past, we can only control how we act right here, right now; and I believe using the past as a lesson in how not to make the same mistakes is sufficient.