How does juxtaposition of light and dark relate to the theme of heart of darkness?

In the Heart of Darkness, light doesn't necessarily symbolize goodness or pure enlightenment. In fact, Conrad's vision is so dark that we're not even sure he fully trusts light. As Marlow says, "sunlight can be made to lie, too" 

Concerto grosso was composed during the classical era. (sorry if i misspelt the name)