What is the theme in Beware of Ruins by A. D. Hope?

Memory and reminiscence is a theme. The word "echoes" is the first indication that "Beware of Ruins" concerns memory. As ruins are a kind of echo of time past, an echo is a kind of ruins of time passing, an original sound decaying. The poem speaks to the problem that memory interprets the past as better than it was, perhaps because it is easier and more pleasurable for the mind. As eyes graze over ruins, trying to imagine them once again intact, so the memory (one's past eyes) grazes over one's past (ruins) and attempts to reconstruct it.