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.
Race and Imperialism are inequalities melting in the same pot. Both emerge out of the idea of superiority. With respect to race, it is the whole race that is believed to be superior whereas with imperialism it is The one Man, being Kurts in this context. Imperialism may be an outcome of racist ideologies, like in Heart of Darkness itself. Where Kurtz believes the savage like Africans are inferior and thus the race he belongs to as superior.
Kurtz is a mere symbol of race and imperialism.. If a man who is brought up in a well to do society comes in contact with uncivilized tribal people, what would he do? The book is probably an answer to this question. It is evident that Kurts strongly believes that he is uplifting the savages from their poor conditions but in the course fails to realise that that's not what he's doing, his power misguides him into falling into the pit of imperialism. Racism leading to imperialism.
The book itself is a struggle between good and the evil. What is good and what is the evil? Kurtz, according to himself is doing good. But he represents racisms and imperialism, how could that be good? They are believed to be evil. Thus, I would say that the issues of racism and imperialism are woven into Heart of Darkness by this struggle of "what is right"