Darkness vs. Light is a major theme. Africa was often thought of as the "Dark Continent" by white Europeans, and there are obvious racial overtones to the issue of darkness. But what is the source of the darkness? Is it the nature of Africa itself, which is portrayed throughout as sinister and foreboding, or is it the influence of Kurtz, and the European imperialism he represents? This theme is closely tied to some that you mention, but it is still, in my opinion, worth expanding on.