Canada

I don't know. You're already getting pretty far into the concept of a subculture. Not an Anthropologist (and it's been a long time since I took Anthropology in University), nor a Sociologist, but as I recall a culture is already sort of a fuzzy concept. It comprises components such as beliefs, languages, behaviours, etc.

You are already starting to make some fine distinctions in order to define an American culture distinct from a broader English-speaking culture, I think. Obviously, there are difference between Australia, New Zealand, Canada, the UK and the USA, but you are already dealing with a largely common language. Although American's might not queue as well, or use the Canadian Sorry, there are some broad similarities that I wouldn't be surprised if some Anthropologists and Sociologists don't consider it a meaningful division between "cultures". At this level, there are plenty of non-white (for some definitions of white) that are a part of this culture. Arguably, predominantly non-white countries are equally a part of this larger global Anglosphere culture. Jamaica, Barbados, etc. Never mind the non-white people in the above mentioned countries. To get to a hypothetical "white" American culture, you are drilling down into some pretty specific territory.

I guess by definition, you could argue that there are "white" American subcultures. The KKK and alt-right would be two of them.

We're getting pretty far down the rabbit hole though. I don't really see a lot of room for a "whites only" North American cultural group that isn't pretty much just the KKK/alt-right/neo-Nazis/whatever. What would be the cultural trappings of this subculture?

I've now given this concept far too much thought.