The framing narrative that surrounds the account of Marlow and his journey to Africa sets the stage for the theme of darkness by focussing us on a literal darkness that lies above London but also the metaphorical darkness that accompanies the imperial enterprise of mankind.
The narrator tells us at the beginning of the book that the air is "condensed into a mournful gloom, brooding motionless over the biggest, and the greatest, town on earth" and again this image is repeated soon after, with the sun's setting described as "stricken to death by the touch of that gloom brooding over a crowd of men" - referring to London. And lastly, before Marlow's interjection, again London is described as "the place of the monstrous town was still marked ominously on teh sky, a brooding gloom in sunshine, a lurid glare under the stars". Despite the narrator's musings on how many adventurers had exited Britain by the Thames and achieved wonderful things, this repeated description seems to suggest that there is a darkness that accompanies such imperial endeavours.
Marlow then interrupts the silence with his first words: "And this also... has been one of the dark places of the earth." In what follows, Marlow associates darkness with being "uncivilised" by describing the reacions of the original Roman invaders when they came upon Britain, saying "there were men enough to face the darkness."
Darkness is thus established as an important theme within the book, and it is significant that the darkness at the end of the book shifts from London to the Thames leads into darkness: "The offing was barred by a black bank of clouds, and the tranquil waterway leading to the uttermost ends of the earth flowed sombre under an overcast sky - seemed to lead into the heart of an immense darkness." The implication is clear - the immense darkness, the capacity for corruption and the capacity to commit heinous crimes is both in man (represented by London, and of course, the character of Kurtz) and in the imperial enterprise.