Any number of literary techniques could be outlined in response to this question. One of the strongest cases, however, could be made for Conrad's use of imagery throughout the novella.
Conrad's descriptions help establish the light/dark motif that is central to the novella's meaning. Darkness obviously plays an essential role in the novella as the title suggests. Darkness is largely metaphorical here and represents an inability to see and a lack of knowledge and insight into situations, people, and the human condition. Light, then, we would assume would represent the opposite. However, the matter isn't so simple. More often, light is merely used as a contrast to darkness not so much as a direct opposite. The emphasis of physical darkness and the diminishing light help to create the dark mood.
In the frame story early in the novella we see the Nellie in the mouth of the Thames, waiting for the tides to take her out to sea and away from London--the "brooding gloom"--and into something far darker yet. It is sunset just as Marlow begins his tale, establishing the motif throughout the novella. It is these descriptions that begin to establish the dark mood.
And as Marlow narrates:
"to understand the effect of [his journey up the Congo River] on me you ought to know how I got out there, what I saw, how I went up that river to the place where I first met the poor chap [Kurtz]. It was the farthest point of navigation and the culminating point of my experience. It seemed somehow to throw a kind of light on everything about me--and into my thoughts. It was sombre enough too--and pitiful--not extraordinary in any way--not very clear either. No. Not very clear. And yet it seemed to throw a kind of light."