:root {
    --bs-body-bg: #ffffff;
    --bs-body-color: #374151;
    --bs-primary: #111827;
    --bs-primary-hover: #1e293b;
    --bs-gray-500: #6b7280;
  }
  body {
    font-family: 'Poppins', sans-serif;
    background: var(--bs-body-bg);
    color: var(--bs-body-color);
    min-height: 100vh;
    display: flex;
    flex-direction: column;
  }
  header {
    position: sticky;
    top: 0;
    background: var(--bs-body-bg);
    border-bottom: 1px solid #e5e7eb;
    z-index: 1020;
    box-shadow: 0 1px 4px rgb(0 0 0 / 0.05);
  }
  .navbar-nav .nav-link {
    color: var(--bs-gray-500);
    transition: color 0.3s ease;
  }
  .navbar-nav .nav-link:hover,
  .navbar-nav .nav-link:focus {
    color: var(--bs-primary-hover);
  }
  .logo {
    font-weight: 700;
    font-size: 1.5rem;
    color: var(--bs-primary);
    user-select: none;
  }

  main {
    flex-grow: 1;
    display: flex;
    justify-content: center;
    align-items: center;
    padding: 4rem 1rem;
    max-width: 480px;
    margin: 0 auto;
    width: 100%;
  }
  .card {
    border-radius: 0.75rem;
    box-shadow: 0 10px 15px rgb(0 0 0 / 0.1);
    padding: 2.5rem 2rem;
    width: 100%;
    background: white;
  }
  h1 {
    font-size: 2.75rem;
    font-weight: 700;
    margin-bottom: 0.5rem;
    color: var(--bs-primary);
    text-align: center;
  }
  p.lead {
    color: var(--bs-gray-500);
    font-weight: 500;
    font-size: 1.1rem;
    margin-bottom: 2rem;
    text-align: center;
  }
  .form-label {
    font-weight: 600;
    color: var(--bs-primary);
  }
  .btn-primary {
    background-color: var(--bs-primary);
    border: none;
    font-weight: 600;
    padding: 0.75rem 1.5rem;
    font-size: 1.125rem;
    transition: background-color 0.3s ease;
  }
  .btn-primary:hover,
  .btn-primary:focus {
    background-color: var(--bs-primary-hover);
  }
  .form-control:focus {
    box-shadow: 0 0 0 0.25rem rgb(17 24 39 / 0.25);
    border-color: var(--bs-primary);
  }
  #message {
    margin-top: 1rem;
    font-weight: 600;
    text-align: center;
    min-height: 1.5rem;
  }
  .input-group-text {
    cursor: pointer;
    user-select: none;
  }