What is the theme of "On Seeing the Elgin Marbles"?

The Elgin Marbles are a famous collection of sculptures from the Parthenon in Athens. They were removed by a British aristocrat, Lord Elgin, and taken back to England in the early nineteenth century. They have been on display in the British Museum in London since 1817 and have been a major source of controversy ever since. Successive Greek governments have demanded the return of the Marbles, seeing them as an integral part of Greek culture and heritage.
However, at the time when the Elgin Marbles were removed, Greece as we know it today was not an independent country. Instead, it was a remote outpost of the Ottoman Empire. When the Marbles first arrived in Britain, many people hailed Lord Elgin for saving precious artefacts of an ancient culture, one of the undisputed foundations of Western civilization.
Certainly, they inspired a sense of awe and wonder in many visitors to the British Museum, one of whom was the poet John Keats. His work "On Seeing the Elgin Marbles" is a paean of praise not just to the fruits of an ancient civilization, but to human artistry in general, a common theme of much of Keats's work. Keats revels in the "glories of the brain," those sublime artistic achievements that have stood the test of time through centuries of ceaseless turmoil and upheaval.
Keats is truly fascinated here, as in "Ode on a Grecian Urn," by the sharply contrasting effects that a work of art can produce. One the one hand, the Marbles are ravishingly beautiful, almost timeless in their frozen beauty; but on the other, they are the product of a particular culture, a particular time and place. And so our attention is irresistibly drawn to the ravages of time that threaten the individual work of art with wear and tear and ultimate destruction.
Keats uses the Marbles as a metaphor for the transience of life in comparison with the eternity of the beautiful. The sheer beauty of the Marbles confronts him with a sense of his own mortality:

So do these wonders a most dizzy pain,
That mingles Grecian grandeur with the rude
Wasting of old time.

The Elgin Marbles themselves may one day decay and crumble into dust. But the beauty that they embody will live on, long after Keats and every single one of us is no more. Ars longa, vita brevis—art is long, life is short.
https://www.poetryfoundation.org/poems/52305/on-seeing-the-elgin-marbles