*{box-sizing:border-box}body{font-family:system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,sans-serif;margin:2rem;background:#f5f5f5;color:#333}h1{color:#2c3e50}nav ul{list-style:none;padding:0;display:flex;gap:.5rem}nav li{padding:.75rem 1.25rem;background:#fff;border:1px solid #ddd;border-radius:6px;cursor:pointer;transition:background .2s}nav li:hover{background:#e0e0e0}nav li.active{background:#2c3e50;color:#fff;border-color:#2c3e50}.message{margin-top:1.5rem;font-weight:500;color:#555}