Dean had mostly been offering just for something to say. If he'd been thinking about it, he probably wouldn't have, since it was just giving a Slytherin an opportunity to poke fun.
The compliment instead was surprising, and he lifted his eyebrows, taking the book back when Theo seemed done and then smiling. "Thanks," he answered. He hesitated and then gave Theo a friendly clap on the shoulder. "Guess we should get back down before Prefects come searching. See you around, Nott." Not that often, usually, but they passed in classes or the halls, often enough.