In the Heart of Darkness the Narrator is connected to the theme of Interiors and Exteriors. “Comparisons between interiors and exteriors pervade Heart of Darkness. As the narrator states at the beginning of the text, Marlow is more interested in surfaces, in the surrounding aura of a thing rather than in any hidden nugget of meaning deep within the thing itself. This inverts the usual hierarchy of meaning: normally one seeks the deep message or hidden truth. The priority placed on observation demonstrates that penetrating to the interior of an idea or a person is impossible in this world. Thus, Marlow is confronted with a series of exteriors and surfaces—the river’s banks, the forest walls around the station, Kurtz’s broad forehead—that he must interpret. These exteriors are all the material he is given, and they provide him with perhaps a more profound source of knowledge than any falsely constructed interior “kernel.”
If you read through all the themes listed on sparknotes you might be able to make more connections than I did.