I think the superhero and Mario sets were also seamless. I don’t mean the heat map gradient but the “soft lines” as @modigliani mentioned. Simple White has one color, but the lines are there. We might also be discussing the same thing and I’m simply unfamiliar with design jargon. 
Anyway, yeah, it would be nice if it were an option to turn on or off. Selling themes with both versions is also possible. I’m unsure whether that idea complements Clear’s simplicity principles though.