Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Hopeful
Theo goes through a whole lot, but the narrator never gets too down about it. He or she maintains a relatively upbeat point-of-view, never commenting that Theo's life is at a dead end or anything of that sort—if anything, the narrator regards misfortunes as just steps in Theo's journey, which may lead to good or bad things. For the person leading us through this book, in other words, it's all a part of life.
Even when Theo seems at his wit's end, though the narrator pauses to describe his misery briefly, he or she—like our main character—pretty much gets right back to the road. Check it out:
Until then, he had believed in his own good nature. He pleaded that he was a kindly, honorable human being. But the bloodied face rose up in front of him. His stomach heaved. He doubled over, retching. He sat on the ground a while, head pressed against his knees. He swore every way he knew: Never again would he do such a deed.
He climbed at last to his feet. The road lay a short distance beyond the field. He set of for it. He did not look back. He did not dare. (3.49-50)
Theo just keeps on trekking, not dwelling on anything for his own sake, and the book does the same. Our narrator could've lingered longer on the horror haunting Theo, but instead he or she fixes their gaze forward, ready to see what comes next.