You know how Charlie Brown's Snoopy, sitting atop his dog house with a typewriter, always starts his stories "It was a dark and stormy night"? Well, he may as well be Emily Brontë, because that sort of mysterious atmosphere sets the tone of the novel. As a Gothic novel, Wuthering Heights is persistently dark and eerie. There is not a very wide range of tones: it's either grim or grimmer.
However, you could argue that the ending has more hope and promise, as do certain moments throughout the story, such as the years after Catherine's death. The attitudes of our narrators help shape the tone as the drama unfolds, so that Lockwood's initial curiosity and fascination convey a lighter feeling than after he realizes how sinister Heathcliff is. Whenever Heathcliff is around, the tone tends to grows darker. Likewise, you can tell Nelly Dean really enjoys storytelling, so she tries to sustain a tone of suspense and mystery – that way she keeps Lockwood's, and by extension the reader's, attention. This is what keeps Lockwood up late and what keeps us reading.