Greetiings
For the Undead I can think of 2 concepts.
1) Seeing the name of their queen, the Undead of this world could have a fantastic Egyptian style as Tomb Kings of Khemri or Vampire Counts (Warhammer).
2) Seeing that they are Undead because they rejected the Ancient Gods and were punished for it, they could have a Steam-punk style aesthetic. Why? They could be an enlightened culture, based on technology or archanotecnology now that they do not have the favor of the gods.
In any case, I don't think that because they are Undead, they want to live in cochambrous ruins or nauseating swamps. They could maintain a lifestyle similar to when they were alive, but themed with darker and gloomier colors.
See you