The "mood" or tone of Conrad's novella is very dark. There is a certain absurdist quality in Marlow's tale that raises the tone somewhat, but for the most part, the story is brooding, mysterious, occasionally violent, and always terrifying.
For example, take the opening of the story, in which Marlow and other representatives of the company are aboard a boat in the Thames at sunset. This is mostly description, but the way Conrad describes the river and its historical connection to Britain's history suggests that the river has been thoroughly "tamed."
The tone is somber and reflective; the men on the boat have almost fallen into a trance. There is a sense that they are in the presence of something far greater than themselves, the "mystery of an unknown earth." When Marlow begins by saying ominously that "this also has been one of the dark places of the earth,” he means that even here, in the supposed heart of civilization, the "darkness" once held sway. What this "darkness" is can't be defined, but it is most definitely felt.
The tone shades into horror in several places. This is the case at the station, where Marlow sees he "grove of death," where broken slaves wait to die. These men are victims of "the work" of the company, emblems of misery. Marlow focuses on a bit of string one of the men wears around his neck, wondering what it could mean. Horror also comes to the fore in the attack on the ship, when Marlow witnesses his pilot on the river boat killed by a spear to the chest. In this case, Conrad chooses to focus on Marlow's reaction, which is a fascination with the blood in his shoes. In both episodes, the tone is heightened by a concentration on mundane detail. The horror is intensified because it is so banal.
The tone shades into the absurd in several places. It is funny, in a dark way, that Marlow has such trouble repairing his boat because of the inability of the company to have the rivets and steel plates shipped to the same location. The accountant's fussy European clothes are absurd and symbolic of the absurdity of the company itself. The shots fired by the ship into the jungle are another example. In each case, the sense is that the forces of capitalism which seek to exploit the jungle are comically incapable of changing it in any meaningful way.