style.css

* {
  margin: 0;
}

header {
  border: 3px solid black;
  padding: 10px 20px;
  align-self: stretch;
}

h2 {
  align-self: center;
}

header nav {
  display: flex;
  justify-content: space-between;
}

nav ul {
  list-style: none;
  display: flex;
  align-items: center;
  font-weight: 600;
}

nav a {
  text-decoration: none;
  color: black;
  font-size: 20px;
  font-weight: 700;
}

nav ul li {
  padding-left: 20px;
  padding-right: 20px;
  cursor: pointer;
}

nav ul li:nth-child(even) {
  border-right: 2px black solid;
  border-left: 2px black solid;
}

.book-item {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding-left: 2%;
  padding-right: 2%;
  margin: 0 auto;
}

.book-item:nth-child(odd) {
  background-color: #dcdcdc;
}

.main {
  display: flex;
  max-width: 100%;
  flex-direction: column;
  justify-content: center;
  align-items: center;
  gap: 15px;
  padding: 10px;
}

.books-list {
  align-self: stretch;
  display: flex;
  justify-content: center;
  flex-direction: column;
  align-items: center;
  gap: 20px;
}

#books-container {
  align-self: stretch;
  margin-left: 5%;
  margin-right: 5%;
}

hr {
  background-color: black;
  width: 15%;
  height: 2px;
}

.add-book h2 {
  align-self: center;
}

form {
  margin-top: 40px;
}

form button {
  float: right;
  background-color: white;
  border-right: 3px solid black;
  border-bottom: 3px solid black;
}

.book-item button {
  background-color: white;
  border-right: 5px solid black;
  border-bottom: 5px solid black;
  margin-top: 2px;
  margin-bottom: 2px;
}

form input {
  border: 2px solid black;
  width: 50vw;
  padding: 3px 8px;
}

.section {
  display: none;
}

.section.active {
  display: flex;
  flex-direction: column;
}

.date {
  font-weight: 600;
  align-self: flex-end;
  padding-right: 10px;
}

footer {
  border: 3px black solid;
  position: absolute;
  bottom: 10px;
  padding: 5px 20px;
  left: 10px;
  right: 10px;
  font-weight: 600;
}

.contact h2 {
  margin-bottom: 40px;
}

.contact p {
  margin-bottom: 40px;
  font-weight: 600;
}

.contact ul {
  font-weight: 600;
}