How are the issues of race and imperialism woven into "Heart of Darkness"?

Both race (or even racism) and imperialism are treated as
the common attitudes of white people at the time the book takes place.  This book is set
on the river Thames around the turn of the 20th century.  Generally speaking, the
Europeans aboard the ship (as well as most other Europeans at this time) are largely
ignorant of the lives of the natives they encounter traveling.  Because these natives
look so very different, the general attitude is that they are sub-human - closer to
animals than they are to humans.  This is evidenced by the repeated referrals of black
people as "niggers," "cannibals," "criminals," and
"savages."

Kurtz's treatise, called the "International
Society for the Suppression of Savage Customs" is yet further evidence of the elitist
attitude carried by white men at this time.  It is one thing to merely refer to those
peoples in passing as less than human - here is a man (and the characters who support
his thinking) who genuinely believes they are harmful to civilized society, so he plans
to educate others to fear them.

Marlow is one of the few
characters whose thoughts pose an opposition to the general attitude of indifference if
not blatant disrespect.  He is often reflecting with sympathy on different situations in
which groups of black men are seen working or enslaved.  His thoughts rarely drive him
to action and even his actions (like sharing the buscuit with the man on his ship) are
as slight as his sympathy - but it is clear the author presents this opposing viewpoint
to remind the audience of the humanity of a group of people who are viewed and mostly
treated, like animals.