Canada

Indeed, even if you were to consider the Quebecois as a distinct subculture, a non-white Francophone is more a member of that culture than a white Anglophone.

Not a sociologist or an anthropologist, but a culture is composed of various cultural trappings, which include customs around food, dress, and language. There's not really a lot of room for a meaningfully defined Canadian culture that somehow excludes people with the wrong shade of skin colour. Butter tarts, Nanaimo bars, the Canadian "Sorry", much of what could be considered Canadian cultural trappings aren't that race based. Except maybe for Residential schools, but some things are better left in the past.

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.