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."
The narration that was alluded to in the previous post is one of the most interesting literary techniques brought out in the novel. Conrad's use of narration is divergent and fascinating. Marlow narrates, but then a third person narrator assists in depicting Marlow's narration. Given the fact that both narrations are driven by the impressions of Kurtz, which is composed by others' views, there is a tapestry of "truth" revealed. In this light, one can understand one of the most thematic elements brought out through its style. The composition of what it means to be "true" and what constitutes "truth" is something that is composed of different elements. There can be no overarching or transcendent conception of truth, but rather snippets of individual perspectives that need to be taken together. This compilation does not yield a clear view, but rather allows a better conception to emerge. It is not black nor white, but much in way of grey. We are left with different impressions of Kurtz, different beliefs of Marlow, and distinct impressions of colonization. None of them are reductive nor easy. This might be why there is a sense of a dark mood to the text, for nothing easy is present. In contrast, it is complex in large part due to the different points of view presented, something that is brought out through the narrative style or literary technique in the work.
Both previous posts are correct in pointing out the dark/light motifs throughout the novella are not absolutes, but fall somewhere in the grey area.
Consider Conrad himself: born in Poland; became a British subject in 1886. By all accounts, his English was so very heavily accented as to be almost un-understandable. The technique of the unnamed narrator telling Marlow's story is Conrad's way of speaking clearly through his writing, rather than speaking out loud.