The most important theme in this story relates to humankind's despair. It is an existential theme of Man against Nature, as existential despair is the result of the nature of life, the nature of existence. Existentialism, briefly, asserts the meaninglessness of life and the burden of having to create one's own meaning. Existential despair relates to the hopelessness of succeeding with this existential task to create individual meaningfulness.
In this story, several characters develop this theme. A man--the deaf old man--with plenty of every material provision, "He has plenty of money," feels such despair that he tries to end his life. A soldier feels the shadow of such despair that he risks arrest to have a few moments of carnal pleasure. The younger waiter feels encroaching despair that he fights off by complaining and by trying to get home to bed to his wife:
I never get to bed before three o'clock. ... He's lonely. ... I have a wife waiting in bed for me.
The older waiter feels the same despair as the deaf old man but he tries to mitigate it for himself and others by providing "a clean, well-lighted place" for them to come to, to sit in the "shadow of leaves" from the glow of the one electric light and maybe forget their despair for a moment. The older waiter grapples with his own despair, which he will not name for himself as he names the old man's despair, by keeping the cafe open late for others who need light (metaphorical and physical light) and by talking about the nothingness of existence (i.e., if life is nothingness, then emptiness belongs to life): "he knew it all was nada." His sleeplessness he calls "insomnia."
He would lie in the bed and finally, with daylight, he would go to sleep. After all, he said to himself, it's probably only insomnia. Many must have it.
A correlated theme explores the value of carnal pleasure in an existentially meaningless life. This is a theme of Human against Self and asks the question, "Can physical happiness fill the meaninglessness and fend off despair?" The implied answer in the story, at least from the older waiter's point of view, is that, no, it can't. He implies this when he says of the old man, "He had a wife once too." In other words, he once had physical pleasure, yet he is now in despair. On the other hand, the younger waiter's implied counter-argument might be that the old man having had a wife, now has despair because she is gone.