Dave Bonta's short poem sets two big ideas side by side: collapse and the end of the day. The speaker begins in a state of wonder, looking at "the whole / design of ruin." That phrasing is striking. Ruin is usually seen as chaos, yet here it seems to have a shape, something that can be studied and even admired. The title, "Decline and fall," points toward that sense of things winding down.
Then the poem shifts to evening, which it calls "the same / but more portable." The poem doesn't spell this out, but the idea seems to be that evening, like ruin, is a kind of ending, only gentler and easier to carry around with us. The last lines compare it to "a quiet pleasure / enlightening the body," which makes the feeling physical and soft rather than gloomy.
The mood is hushed and reflective. The speaker doesn't mourn what is falling apart. Instead, there's a calm acceptance, even a faint warmth, in noticing that endings can be quietly beautiful.