Heart of Darkness: Theme Analysis

The Pervasiveness of Darkness
Perhaps the strongest theme in the novel is that of darkness.  Indeed, darkness seems to pervade the work.  Marlow's tale begins and ends in literal darkness; the setting of the novel is often dark, such as when the steamboat is socked in by fog or when Marlow retrieves Kurtz; dark-skinned individuals inhabit the entire region; and, of course, there is a certain philosophical darkness that permeates the work.  But within the tale darkness operates in several ways. 
As any child knows, darkness symbolizes the unknown; it gains its power from its ability to conceal things we are too frightened to face.  Several times in the novel we see characters afraid, not of the darkness itself, but of that which potentially lies within it.  One of the most alarming scenes occurs when the men aboard the fog-bound steamer hear a shrill cry from somewhere around them.  It is particularly frightening because the men know some potential threat is near, but they cannot see it; it is simply out there in the darkness, waiting. 
Darkness also effectively conceals certain savage acts.  It is possible to operate in the cover of region's darkness in ways that would not be possible in the more civilized Europe.  For example, when the Manager suggests that the "scoundrel," who is suspected of helping Kurtz procure his ivory, should be hanged as an example, his uncle agrees, noting that such actions are possible in the Congo, a region far from the "light" of civilized action.  And Kurtz's most disturbing act, the placement of human heads atop poles surrounding his station house, is only possible in the concealed Congo. 
Of course, darkness is also very compelling.  Despite the fear it induces, there are plenty of men who are willing to brave it for its potential rewards.  For the company men, the incentive is material wealth in the form of ivory.  There are, however, other rewards.  Marlow travels to the region because of a map he sees, which lists the area as one of the few largely uncharted lands left.  To him, the Congo is a place to undertake a great adventure.  The Harlequin is a physical and spiritual wanderer, and through Kurtz and his dark station, his mind has been "enlarged" he has found a sense of purpose.  The character who most fully embraces the darkness is, of course, Kurtz.  He has been completely transformed by his experience in the Congo.  He has looked deeply within himself and has seen his own potential for savagery, yet he has accepted it. 
The Europeans try to push back the darkness, if only temporarily, through their white clothes, adherence to European customs and morals, and technological advances, like the steamboat and the railroad.  But the novel argues that the darkness is too enveloping.  In the preface to his tale, Marlow remarks that London was once "one of the dark places of the earth." Later he sees how quickly the jungle reclaims its territory.  When he locates the remains of his predecessor, Captain Fresleven, who died in an argument with a native chief, he notes that "the grass growing through his ribs was tall enough to hide his bones." These remarks suggest that in time Europe too will be reclaimed by wild.  The light of civilization with someday return to darkness. 
Colonization as Destruction
Another major theme in the novel is the notion of colonization as a destructive, rather than constructive, force. 
Kurtz's initial approach to colonization is very altruistic; he believes that each company station "'should be like a beacon on the road toward better things, a center for trade of course but also for humanising, improving, instructing.'"
Kurtz is not alone in this philosophy.  The International Society for the Suppression of Savage Customs, which commissions Kurtz to write a report, is likely an organization that believes in "civilizing" the inhabitants of Congo.  Even Marlow's aunt, who helps to secure his position, is pleased that her nephew will help in "weaning those ignorant millions from their horrid ways."
Of course, the reality of colonization is very bleak.  As Marlow comments: "The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much." Marlow sees firsthand the cold truth of colonization: physically wasted workers operating in deplorable conditions, backstabbing co-workers jockeying for the most profit and recognition, and a colonized people literally being shackled.  It's as if the company is a steamroller plowing through the jungle, flattening anything and anyone that happens to be in the way, all, of course, in the name of profit. 
The Manager condemns Kurtz for his "unsound" methods, yet in one sense Kurtz has achieved the ultimate form of colonization: the natives actually worship him.  As a result, he brings in the most ivory.  Of course, it is at Kurtz's station where Marlow sees the greatest act of savagery, the placement of the decapitated heads of "rebels" atop poles.  By the time Marlow encounters Kurtz, Kurtz no longer has any noble intentions; instead he feels the need to "Exterminate all the brutes!'" 
Colonization may help to maintain the surface luster of the home country, but
there are no benefits for those being colonized, only hardship, suffering, and death.