There are many themes in Conrad's work. The idea of exposing what lies beneath the veneer of order and calm is a very compelling one. Kurtz is seen as pillar of control and "civilization" prior to his descent into the Congo. Once immersed, though, he gives into his "darker" nature through the use of violence, cruelty, and absolute "horror." The reflection of what lies beneath the surface seems to be a very strong indictment of European colonialism, something depicted as civilized forces helping out "the savages." Through Kurtz, the reader understands that individuals, regardless of ethnicity, can possess the propensity to act in a savage and horrific manners. The overriding theme through Kurtz is that individuals possess a side of their own sense of reality that lies outside the confines of society and to better understand it is to help better grasp a part of our own senses of self.
One of the primary themes of the novel is exemplified in its title: the darkness of the human heart. Marlow's journey into the heart of Africa gives him the opportunity to learn much about human nature, and Conrad offers a number of themes. Foremost, however, among them seems to be man's capacity for evil.
From the onset of his arrival in Africa, Marlow is shocked to see the chaos and cruelty of the Outer Station where work seems to serve no purpose and the natives are dying of overwork, starvation, and neglect. Who is responsible? The Europeans have come to Africa, ostensibly as Marlow's aunt believes, as "emissaries of light" to spread civilization among the "savages" but in reality to get wealthy from the abundant ivory, and they will stop at nothing to succeed. As his journey continues upriver, Marlow feels the influences of his experiences as he says he was becoming "scientifically interesting," evoking the Belgian doctor's assertion that those who go to the Congo change "inside," and even "savage." When Marlow finally reaches the Inner Station in search of Kurtz, he is appalled to discover the depths of Kurtz's depravity. Kurtz has "taken a seat among the high devils of the land" and engaged "in unspeakable rites" with the natives. Any veneer of civilization this "universal genius" had has been stripped away as Kurtz has succumbed to the lure of ivory and wealth, sinking so far he has even erected the heads of rebels on poles around the Inner Station.
Conrad uses a number of key symbols. Look for specific items that he carefully describes such as the oil painting in the Brickmaker's hut. The Brickmaker himself is called a "papier-mache Mephistopheles." Notice the many references to light and dark, to white and black; Conrad often uses these as symbols in a way opposite to the their traditional symbolic values.