The theme of this poem is the fragility of life and the unexpectedness of death. The title of the poem is an allusion to Shakespeare's play, Macbeth. In this play, Macbeth has just learned that his wife is dead and he laments her early death. He says, "out, out brief candle." The candle is a symbol of life -- in this case the life of the boy. Candles are finite and their flames are easily extinguished, just like the boy in the poem. Even though everyone knows that humans will all die at some point, no one in the poem expected that this would be the boy's last day, but he died in a slow instant -- his life was blown out like a candle.
To me, the theme of this poem is that little decisions can have huge consequences. If they had stopped working a little earlier, they would not have needed to be called in to supper. If the girl had been a little more careful, perhaps, the boy would not have been surprised when she called him to supper. Small things like that end up leading to the loss of the boy's life.