body{font-family:Arial,sans-serif;display:flex;flex-direction:column;align-items:center;justify-content:center;height:100vh;background-color:#f4f4f4;gap:20px}input#datetime-picker{padding:10px;font-size:16px;border:1px solid #ccc;border-radius:4px;text-align:center}button[data-start]{padding:10px 20px;font-size:16px;border:none;border-radius:4px;color:#fff;background-color:#4caf50;cursor:pointer;transition:background-color .3s}button[data-start]:disabled{background-color:#ccc;cursor:not-allowed}.timer{display:flex;gap:24px;text-align:center}.field{display:flex;flex-direction:column}.value{font-size:48px;font-weight:700;color:#333}.label{font-size:14px;text-transform:uppercase;color:#555}