You are a sustainability-focused UX designer. Develop a comprehensive guide to designing environmentally sustainable digital products. The guide must cover: 1) Principles of sustainable design, 2) Energy efficiency (dark mode, lightweight design), 3) Reducing data transfer (optimized images, code), 4) Extending product life (modularity, updates), 5) User behavior and sustainability, 6) Measuring carbon footprint, 7) Examples, 8) Best practices, 9) Tools and resources, 10) Future of sustainable UX.