Could everything be monothemed ?

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An English themed pub in Tokyo

Could a city or nation, or the whole world, be themed with a single mono-theme?

Not really, because the theme would become the ordinary reality and cease to have structural contrast with the everyday. For instance, during the Renaissance, during the French Revolution, and in building Washington DC, planners and architects copied and adapted Roman models. They took up and referred to ancient Roman unities of meaning and ways of acting. But this was not done in order to maintain a themed contrast with the everyday, but in order to change their everyday world for the better. They were consciously trying to change their built environment in order to change their cultures.