"Spartan" is an adjective derived from Sparta, an ancient Greek city-state famous for its austerity. The adjective is used today to mean basic and minimal. I'm looking for an antonym, i.e. a culture that implies ornate luxury. The antonyms I'm finding are words like "opulent" or "embellished", which aren't based on a culture or place. Is there an adjective based on a culture or place that implies the opposite of Spartan?
"Spartan" is obviously a reference to the city of Sparta; if you want an antonym that keeps the geographical atmosphere, then I think sybaritic is the word you're looking for:
pertaining to or characteristic of a sybarite; characterized by or loving luxury or sensuous pleasure
of, relating to, or characteristic of Sybaris or its inhabitants.
Sybaris (Ancient Greek: Σύβαρις; Italian: Sibari) was an important city of Magna Graecia. It was situated on the Gulf of Taranto between two rivers, the Crathis (Crati) and the Sybaris (Coscile).
The city was founded in 720 BC by Achaean and Troezenian settlers. Sybaris amassed great wealth thanks to its fertile land and busy port. Its inhabitants became famous among the Greeks for their hedonism, feasts, and excesses, to the extent that "sybarite" and "sybaritic" have become bywords for opulent luxury and outrageous pleasure-seeking.
If a Spartan eschews pleasure for the sake of pleasure, then an Epicurean pursues pleasure for the sake of pleasure.
Mind you, "Epicurean" doesn't directly reference a culture. However, it does reference a philosophical or aesthetic bent. "Spartan" can also reference a philosophical or aesthetic bent.
Check the general references to see whether that word suits your purpose.
Maybe Byzantine would match your need. It is related in culture and can mean complex and intricate in contrast to simple and straightforward.
Baroque was a highly decorative art style \ movement that was definitely not spartan. Specifically "late baroque" or "rococo".
Rococo artists and architects used a more jocular, florid, and graceful approach to the Baroque. Their style was ornate and used light colours, asymmetrical designs, curves, and gold.
The term Bourgeois comes to mind:
In political economy, political philosophy, sociology, social sciences, and history, the bourgeoisie is the wealthy stratum of the middle class that originated during the latter part of the Middle Ages (AD 476–1453).