Can you think of any particular lines in To the Lighthouse where the diction builds tone, and in turn, content?
Well, there are plenty of examples of this in the novel. Woolf's style of stream-of-consciousness lends itself well to the use of diction to build tone and content. You might like to think about the following quote as a good example of what you are talking about, however:
Who shall blame him? Who will not secretly rejoice when the hero puts his armour off, and halts by the window and gazes at his wife and son, who, very distant at first, gradually come closer and closer, till lips and book and head are clearly before him, though still lovely and unfamiliar from the intensity of his isolation and the waste of ages and the perishing of the stars, and finally putting his pipe in his pocket and bending his magnificent head before her—who will blame him if he does homage to the beauty of the world?
This quote comes from Chapter 6 of "The Window," and shows us the internal thoughts of Mr. Ramsay as he walks across the grass and sees his wife and James in the window. Mr. Ramsay is shown to understand the ephemeral nature of humanity and the way that death will negate his life completely, and that intellectual immortality is a holy grail which many men seek but only a minority find. The diction of the passage above indicates the way that all things, including both the benefits of his career and the stars in the sky, are fated to perish. In response to this rather depressing fact, Mr. Ramsay takes consolation from the beauties of nature that surround him. Seeing his son and wife together enables him to soothe his fears and thoughts of oblivion. Bringing his wife and son "closer and closer" to him allows him to cling harder to the beauty of life and save himself from despair.