:root{color-scheme:light dark}body{font-family:system-ui,-apple-system,Segoe UI,Roboto,sans-serif;margin:0;padding:2rem}.container{max-width:720px;margin:0 auto}button{padding:.6rem 1rem;font-size:1rem;cursor:pointer}.card{margin-top:1rem;padding:1rem;border:1px solid #ccc;border-radius:8px}.error{border-color:#e00;color:#e00}footer{display:flex;justify-content:center;padding:5px;background-color:#45a1ff;color:#fff}