What is the theme in ode on a grecian urn?

A main theme of Keats' Ode to a Grecian Urn is how looking at the art of a lost era puts us in touch with how people thought and felt in times now long gone.
There is a generalisation of this idea in the last lines:

Beauty is truth, truth beauty,

Keats seems to be saying that although people and cities and even civilisations all die, the beauty that they create lives after them - and achieves a sort of permanence.
Keats was very concerned with what - if anything - of a human life survives death. His mother had died of tuberculosis in 1810, and his brother in 1818. Keats had also seen much illness and early death in his work as an apprentice surgeon. And Keats himself would die young of tuberculosis, though in 1818 he probably didn't know that yet.