:root {
    --white: #fffbfb;
    --grey: #f0eef1;
    --black: #050505;
    --red: #ff7070;
    --light-green: #9fff9c;
    --light-red: #ff9c9c;
    --border-radius: 8px;
    --spacing-xs: 5px;
    --spacing-sm: 10px;
    --spacing-md: 15px;
    --spacing-lg: 20px;
    --spacing-xl: 40px;
    --container-width: 1200px;
    --shadow: rgba(0, 0, 0, 0.16) 0px 1px 4px;
  }
  
  /* CSS RESET */
  
  *,
  *::before,
  *::after {
    box-sizing: border-box;
    padding: 0;
    margin: 0;
  }
  
  html {
    /* footer support */
    position: relative;
    min-height: 100%;
  }
  
  body {
    background-color: var(--grey);
    color: var(--black);
    font-family: 'Poppins', sans-serif;
    font-size: 18px;
    font-weight: 500;
    word-wrap: break-word;
    /* footer support */
    margin-bottom: 100px;
  }
  
  button,
  input {
    border: none;
    color: inherit;
    font-family: inherit;
    font-size: inherit;
    font-weight: inherit;
    cursor: pointer;
    outline: none;
  }
  
  input[type='text'],
  input[type='number'] {
    cursor: text;
  }
  
  /* UTILS */
  
  .container {
    max-width: var(--container-width);
    padding: var(--spacing-lg);
    margin: 0 auto;
  }
  
  .btn {
    padding: var(--spacing-sm) var(--spacing-md);
    border-radius: var(--border-radius);
    transition: filter 0.15s ease-in-out;
  }
  
  .btn:hover {
    filter: brightness(90%);
  }
  
  .btn-add {
    font-size: 24px;
    font-weight: 600;
  }
  
  .btn-light-green {
    background-color: var(--light-green);
  }
  
  .btn-light-red {
    background-color: var(--light-red);
  }
  
  /* HEADER */
  
  .header {
    background-color: var(--white);
    box-shadow: var(--shadow);
  }
  
  .header .container {
    display: flex;
    align-items: center;
    justify-content: space-between;
  }
  
  .logged-in,
  .logged-out {
    display: none;
  }
  
  .logged-in.active,
  .logged-out.active {
    display: flex;
    gap: 20px;
  }
  
  /* MAIN */
  
  .main {
    margin-top: var(--spacing-lg);
    text-align: center;
  }
  
  .books-grid {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
    gap: var(--spacing-xl);
    margin-top: var(--spacing-xl);
  }
  
  .book-card {
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    font-size: 20px;
    gap: var(--spacing-lg);
    padding: var(--spacing-lg);
    border-radius: var(--border-radius);
    background-color: var(--white);
    box-shadow: var(--shadow);
    line-height: 1.2;
  }
  
  .button-group {
    display: flex;
    flex-direction: column;
    gap: var(--spacing-lg);
  }
  
  /* FOOTER */
  
  .footer {
    position: absolute;
    bottom: 0;
    left: 0;
    display: flex;
    align-items: center;
    justify-content: center;
    gap: 10px;
    width: 100%;
    padding: var(--spacing-sm);
  }
  
  .fa-github {
    font-size: 24px;
    color: var(--black);
    transition: transform 0.3s ease-in-out;
  }
  
  .fa-github:hover {
    transform: rotate(360deg) scale(1.2);
  }
  
  /* MODALS */
  
  .modal {
    position: fixed;
    z-index: 1;
    top: 50%;
    left: 50%;
    width: 300px;
    padding: var(--spacing-lg);
    border-radius: var(--border-radius);
    background-color: var(--grey);
    text-align: center;
    transform: translate(-50%, -50%) scale(0);
    transition: 0.2s ease-in-out;
  }
  
  .modal.active {
    transform: translate(-50%, -50%) scale(1);
  }
  
  .add-book-form {
    display: flex;
    flex-direction: column;
    align-items: center;
    gap: var(--spacing-lg);
  }
  
  .add-book-form button {
    width: 100%;
  }
  
  .error-msg {
    display: none;
    color: red;
  }
  
  .error-msg.active {
    display: block;
    margin-top: -15px;
  }
  
  .input {
    width: 100%;
    padding: 10px;
    border-radius: var(--border-radius);
  }
  
  .is-read {
    display: flex;
    gap: var(--spacing-md);
  }
  
  .checkbox {
    width: 20px;
    height: 20px;
    margin-top: 4px;
    accent-color: white;
  }
  
  .overlay {
    position: fixed;
    top: 0;
    left: 0;
    display: none;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.5);
  }
  
  .overlay.active {
    display: block;
  }
  
  /* LOADING RING */
  
  .lds-dual-ring {
    display: none;
    width: 38px;
    height: 38px;
  }
  
  .lds-dual-ring:after {
    content: ' ';
    display: block;
    width: 30px;
    height: 30px;
    margin: 8px;
    border-radius: 50%;
    border: 3px solid var(--black);
    border-color: var(--black) transparent var(--black) transparent;
    animation: lds-dual-ring 1.2s linear infinite;
  }
  
  .lds-dual-ring.active {
    display: block;
  }
  
  @keyframes lds-dual-ring {
    0% {
      transform: rotate(0deg);
    }
    100% {
      transform: rotate(360deg);
    }
  }
  
  /* MEDIA QUERIES */
  
  @media (max-width: 400px) {
    .header .container {
      flex-direction: column;
      gap: var(--spacing-md);
      padding-top: var(--spacing-sm);
    }
  
    .main {
      margin-top: 0;
    }
  
    :root {
      --spacing-xl: 20px;
    }
  }