Your original question asked more than one question, so I have edited it and instead of focussing on racism I have chosen to focus on colonialism, which is in a sense the major theme of this excellent novella and incorporates racism.
This is a massive topic so I can only hope to begin to scrape the surface in this response. Critics are sharply divided about whether this text reflects Conrad's own ambivalent attitudes towards colonialism, but it is clear that it does ask serious questions about the value of white civilisation and the desirability of its transplantation to what were then considered as "primitive" or "developing" countries. It is important to note that at the beginning, by referring to the Roman conquest of Britain, colonialism is shown to have existed as long as humanity itself has existed, and it still prevails in one form or another. What is interesting about this novel is that it presents colonialism not simply as an economic and political venture only, but also as a consequence of the individual's lust for power and possessiveness and even as an epitome of man's capacity for evil.
Note how towards the beginning of the book we are given glimpses of the "dark side" of colonialism, with the man-of-war firing into the empty continent, which Marlow says has a "touch of insanity". He described the colonial expedition as a "merry dance of death and trade" and a "weary pilgrimage amongst hints for nightmares." As he penetrates ever deeper into the "heart of darkness" or Africa, he discovers more and more of the true horrors of colonialism, as he ventures upon the "grove of death" for example, and then finally meets Kurtz. Surely, Marlow's assessment of colonialism in the first few pages of the novel is born out:
The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, it not a pretty thing when you look into it too much.
What the novel reveals however is the danger of creating a larger narrative of redemption, education and moral superiority that justifies the racism, abuse of power and acts of torture that we are presented with in the novel.