A multi-themed world ?

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Multiple themes along Disney's Epcot lakefront

Could an entire city or nation, or world, be themed, but with multiple themes?

It is possible, but the themes would not be stable. The theme effect demands contrasts to everyday life. If everything is themed, but with different themes in different places, then the ordinary would be that there was always some theme. The themes would be "what you do": You have to present yourselves as Roman here, and as a Disney character there. It would be difficult for such a life to remain themed, since it would soon become just a set of appropriate social roles or actions for various places. Aristocratic behavior at a party or lower class behavior at a bar are not themes but grammatically normative roles and attitudes.