Joseph Conrad's Heart of Darkness is set primarily in Africa and the narrator is of European descent, so of course there is the element of race in this story. Marlow does not seem to be any more or less racist than anyone else, which indicates a prevailing attitude of racism rather than a particular prejudice by one or two people. While the whites consistently refer to the black natives with pejorative and ugly names, they do not speak out of anger or derision. In fact, Marlow says he feels a thrum of connection to these wild, dancing, gesticulating people. Instead, there is a sense that the whites see the blacks merely as undeveloped humans--nothing to scorn but also nothing to particularly admire or appreciate. This is best demonstrated by Marlow's willingness to give a dying black man a biscuit and then casually dismissing the death of his black helmsman as
a savage who was no more account than a grain of sand in a black Sahara.
Though he misses the man's function on his ship, he does not mourn for the man because he is black and therefore not worth mourning. Ironically, of course, so many of the black natives are more moral and honest people, despite their savage ways, than the white, Imperialist encroachers. The issue of race, value, and human worth is one theme in this story.
Another important theme in this short novel is madness. There is something about this mysterious, hot, and thrumming continent which is enough to create disorder in the minds of Marlow and his men. As early as chapter one, Marlow feels this:
The idleness of a passenger, my isolation amongst all these men with whom I had no point of contact, the oily and languid sea, the uniform sombreness of the coast, seemed to keep me away from the truth of things, within the toil of a mournful and senseless delusion.
When prolonged isolation is added to this, we get Kurtz, who has literally gone insane after years in the African jungle. His dying words reveal his delusional sense of ownership of things which cannot be owned:
"You should have heard him say, 'My ivory.' Oh, yes, I heard him. 'My Intended, my ivory, my station, my river, my—' everything belonged to him."
Whether it is the effect of the jungle or his unchecked greed and power, he dies believing he, like God, possesses everything he sees. That is insanity.
A final theme among many others (such as corruption, deception, communication, and violence) is the quest for truth. Marlow is searching for something important and true and worthy of emulation; to find it, he must endure trials and testings just as anyone on a quest must do. He is looking for something morally perfect and righteous, not the "flabby rapacious folly" of the others who have imposed their imperial will on the natives.
Of course the object of his quest is Kurtz, but what he eventually discovers is that Kurtz is more evil, greedy, and cruel than anyone else Marlow has met or even heard about. This revelation is devastating, for he realizes that the core, the heart, of everything "lead[s] into the heart of an immense darkness.” What he thought was true was a lie, and now he must even lie to the Intended to keep her from knowing the one inescapable truth about Kurtz. Truth does not exist for him now.
This is a short work, but it is full of lessons to be learned and realizations to be made. Most of them center around who Kurtz is (or has become) and how Marlow reacts to this this place and this man.