The tone in any type of writing represents the attitude towards the topic. We might expect anger or even sympathy in this type of book but the author lacks this type of emotion. Instead, the tone is bleak. The author is almost submissive just as Winston appears to be submissive.
Even when a character seems to have some hope, as a reader, you realize there is no hope. Even the reader has to realize that O'Brien is most likely loyal to the party and Winston will ultimately lose his struggle. You almost fear for Winston and know the hope he has will be short-lived and unrealized. Even when we reach the end, again the main character has no choice but to be submissive.
The tone is bleak because the purpose here is to be a warning as to what can happen when too much control is given to a government. It is a bleak situation and to show that there is hope would be to imply this type of government is not as dangerous and Orwell wants you to understand it is.
In regards to detracting from the characters' discourse, again, the tone is bleak because the hope is bleak. The tone foreshadows the inevitable ending.
The overall tone of 1984 is one of unrelieved darkness, unless one is a very careful reader. Then, one will realize that the Party's rule must have been destroyed soon after the death of Winston Smith.
Throughout, Orwell underlines the hopelessness of Winston's struggle against the Party. Time and time again -- in his keeping a diary, in his prowling, and most notably in his sex with Julia -- Winston takes risks that make no sense if his primary goal is to destroy the rule of Big Brother. In fact, he wilfully blinds himself to the traps he walks into. I suspect, for instance, that many readers realize very early that O'Brien is utterly true to the Party; Winston has trouble recognizing this even when O'Brien is torturing him.
The tone of the main narrative is thus very black. Winston is not a sympathetic protagonist -- he is himself part of the apparatus of oppression, and he promises to ignore moral standards in struggling with the Party -- and the novel ends depressingly with him crushed.
But that is not the end of the story. Orwell always insisted the appendix on Newspeak was critical. Why?
On examination, we discover that the appendix is written in the subjunctive and simple past -- the way we speak about things that did not happen, or that are no longer happening. Orwell's ironic touch of hope, and the book's cleverest feature, is that the Party that placed its faith in language is marked as doomed through the subtle use of language.
The author uses a bleak tone, whereby he has no more hope regarding the desperate struggle between Winston and Big Brother.