body { font-family: Arial, sans-serif; background: #f4f4f4; margin: 0; padding: 0; }
.screen { padding: 20px; max-width: 600px; margin: auto; }
header { display: flex; justify-content: space-between; background: #333; color: #fff; padding: 10px; }
nav { margin: 10px 0; }
button { margin: 5px; padding: 10px; }
.app-section { padding: 10px; background: #fff; margin-top: 10px; }