Darkness in this short story symbolizes despair while light, especially when accompanied by cleanliness and order, symbolizes a refuge from the chaos and nothingness ("nada") of the universe.
In this story, an old man sits in "the shadow the leaves of the tree made against the electric light" of a cafe. This is a pleasant setting, though the shadows reflect the sadness that hovers over the old man's life. We learn that the old man recently tried to hang himself because of despair.
The light of the cafe provides the old man with a bulwark against his despair. This light represents safety. The younger waiter is angry that the old man is lingering late in the cafe. The younger waiter wants to get home to his wife. However, the older waiter understands why the old man stays, even though it is very late at night. He feels a sense of compassion for the old man, with whom he identifies.
The light of the cafe stands for the light of compassion. In a meaningless post-World War I world, islands of compassion and light like the cafe are important to keeping despair at bay. A clean, well-lit cafe may seem like very little in the larger scheme of things, but it is these little refuges that make life bearable.