Style built-in shelving by painting them in deep, moody colors like emerald green or charcoal. Arrange books with spines facing outward, intermixed with antique artifacts and vintage collectibles. Add layered lighting such as sconces or accent lamps to highlight shelves. Use leather-bound volumes, globes, and brass details for authentic dark academia vibes. Keep styling intentional to create warmth and old-world charm in your built-ins.