Probably one of the main themes is the struggle between good and evil. As the title implies, the characters might be falling into darkness, which means they are likely being tempted by evil forces or their own dark desires.
Well, it could be centered around the idea of hope in the face of impending doom which is symbolized by the 'darkness'. Maybe the characters in the novel are fighting to keep their world from being consumed by this 'darkness', whether it's a physical, moral or spiritual darkness. They might be trying to uphold values like light, goodness and order.
One possible theme could be survival. If darkness represents a great danger, then the characters in the stories would be focused on surviving it. Another theme might be hope. Despite the impending darkness, there could be characters who still hold onto hope and strive to make things better.
I'm not sure specifically as I haven't read it, but it could be a hero figure who is trying to prevent the darkness from falling. There might also be a villain or some sort of antagonist that represents the forces of this 'darkness'.
One of the main themes is the corrupting nature of power. In the novel, the political system twists and distorts the characters' values. Another theme is the loss of individual freedom. The main character is trapped in a system that controls every aspect of his life. Also, the idea of betrayal is prominent, as characters turn on each other in the name of political ideology.
One of the main themes could be the struggle against the unknown in the darkness. It might also touch on themes of survival and the human spirit's resilience.
The themes include the exploration of good and evil. The setting in the Congo makes the characters face their own morality. Racism is also a theme. The Europeans in the novel often look down on the African natives, which reflects the prevalent racist attitudes of that time. And there is the theme of isolation. Marlow is isolated during his journey, both physically in the African wilderness and psychologically from the people around him, which makes him question his own values.
One of the main themes is adventure. The characters are constantly on quests and facing various challenges in a fantasy world. Another theme is friendship. Despite their differences, the characters like Kazuma, Aqua, Megumin and Darkness form a unique bond as they journey together.
Not having read 'The Darkness' makes it a guess, but many Icelandic novels explore themes like the power of the Icelandic landscape. It could be that in 'The Darkness', the harsh and unforgiving Icelandic environment plays a big role, perhaps representing the internal struggles of the characters. Also, themes of identity, both personal and cultural, are often explored in Icelandic literature, so that might be in there too.
One of the main themes is imperialism. The novel shows the dark side of European imperialism in Africa, how it exploited the land and the people. Another theme is the loss of innocence. As the protagonist travels deeper into the Congo, he witnesses so much horror that his own sense of self and innocence is shattered.
Another theme is the coexistence of hope and despair. In many such novels, even in the darkest of times, there are glimmers of hope. Take 'The Hunger Games' for instance. The Capitol represents the darkness with its oppression, but Katniss's defiance and the hope of a better future for the districts are the light. This theme often makes the story more complex and engaging.