I like white dialogues - I agree with @frederik-f, dark ones can look creepy.
I am less sure about the style difference between the app dialogues and shell dialogues:
For shell dialogues, differences are: no dark stripe at the top; borderless buttons; rounded bottom corners; dimming of the desktop. I think both dialogue styles look very good but probably have a preference for the app ones, because they look more “Communitheme” to me (the shell dialogues are nice but look alien).
It may actually be desirable to have a clear visual distinction between shell dialogues and app dialogues. But I don’t IMHO think the points of distinction should be arbitrary design choices (e.g. one has square corners at the bottom and one has rounded ones… one has button style “x” and one has button style “y”, etc.).
Of the differences listed, I think the one that makes most logical sense is dimming the desktop for shell dialogues, because it emphasises a shift of focus away from apps, open documents, etc., so you can talk to the shell itself.
Are there limitations in terms of what can and can’t be themed here? I think I would like to see the shell dialogues styled in a similar way to the rest of Communitheme, if possible. A dark bar at the top would give a nice balance between light and dark (so the dialogues are light but not too “bright-seeming”) and the desktop could still be dimmed for shell dialogues only, to distinguish them from app dialogues: