index.css

:root {
  --bg-color: hsl(0, 6%, 90%);
  --txt-color: hsl(180, 87%, 12%);
  --odd-color: hsl(240, 0.6%, 30.8%);
  --even-color: hsl(0, 0%, 100%);
  --font-fam: 'Poppins', sans-serif;
  --light-blue: hsl(240, 100%, 50%);
}

html {
  box-sizing: border-box;
  font-size: 100%;
}

*,
*::after,
*::before {
  box-sizing: inherit;
  padding: 0;
  margin: 0;
}

body {
  font-family: var(--font-fam);
  background-color: var(--bg-color);
}

a {
  text-decoration: none;
  color: hsl(0, 0%, 0%);
}

/* header styles  */
.header {
  display: flex;
  justify-content: space-between;
  padding: 18px;
  background: white;
  box-shadow: 1px 1px 10px rgba(0, 0, 0, 0.2);
  position: fixed;
  width: 100%;
  margin-bottom: 24px;
  top: 0;
}

form ul li {
  list-style-type: none;
}

.header li {
  list-style: none;
  margin-right: 41px;
}

@media (max-width: 768px) {
  .header li {
    margin-right: 12px;
  }
}

.logo {
  font-weight: 500;
  font-size: 21px;
}

.header-nav ul {
  display: flex;
}

.footer {
  position: fixed;
  bottom: 0;
  width: 100%;
  padding: 10px 31px;
  box-shadow: 1px 1px 13px rgba(0, 0, 0, 0.1);
  background: white;
}

.main {
  margin-top: 79px;
}

.display-date {
  text-align: right;
  margin-right: 30px;
}

.contact-container .contact-title {
  text-align: center;
  font-size: 29px;
  margin-top: 36px;
}

.contact-container .description {
  width: 70%;
  margin-inline: auto;
  margin-top: 22px;
}

.contact-container .description ul {
  margin-top: 21px;
  margin-left: 38px;
}

/* form styles  */
.space-inputs {
  width: 100%;
  padding: 10px 4px;
  font-size: 14.5px;
  font-family: 'Poppins', sans-serif;
  margin-bottom: 10px;
  padding-left: 10px;
}

.space-inputs:focus {
  border: thin solid green;
  outline: none;
}

form {
  margin-block: 8px;
  margin-inline: 10%;
}

.form-title,
.disp-title {
  text-align: center;
  font-size: 29px;
  margin-block: 12px;
}

.add-btn {
  display: block;
  margin-left: auto;
  padding: 6px 20px;
  font-size: 16px;
  color: var(--txt-color);
  font-family: var(--font-fam);
  font-weight: 500;
  cursor: pointer;
  box-shadow: 1px 1px 3px black;
}

.book-container {
  display: flex;
  justify-content: space-between;
  padding: 10px;
  background-color: var(--even-color);
}

.book-container:nth-child(odd) {
  color: white;
  background-color: var(--odd-color);
}

.book-container button {
  color: var(--txt-color);
  padding: 2px;
  font-family: var(--font-fam);
  font-weight: 500;
  cursor: pointer;
  box-shadow: 1px 1px 3px black;
}

@media (min-width: 768px) {
  .book-container button {
    padding: 4px;
  }
}

.blue {
  color: var(--light-blue);
  font-weight: 500;
}

/* error messages */
.error-title,
.error-author {
  font-size: 12px;
  color: red;
  margin-left: 14px;
  margin-top: -6px;
  margin-bottom: 4px;
}

/* handle popup */
.popup {
  width: 300px;
  background: #f1d4d4;
  margin-inline: auto;
  border-radius: 6px;
  text-align: center;
  padding: 16px;
  box-shadow: 1px 1px 9px rgba(0, 0, 0, 0.3);
}

.disp-container {
  margin-inline: 28px;
}