index.css

.library {
  margin: 0 auto;
  max-width: 700px;
  border: 1px grey solid;
  margin-bottom: 20px;
  display: flex;
  justify-content: center;
  flex-direction: column;
}

.library > :nth-child(odd) {
  background-color: lightgray;
}

.bookdiv {
  display: flex;
  justify-content: space-between;
  align-items: center;
  padding: 15px;
  border-bottom: 1px solid gray;
}

.library-section {
  text-align: center;
  padding: 20px;
  margin-top: 15px;
}

.navbar {
  background-color: #e3f2fd;
}

.active {
  display: block;
}

main {
  margin-top: 70px;
}

.page-footer {
  margin-top: 50px;
  background-color: #e3f2fd;
}

.nav-link:hover {
  background-color: blue;
}

.date {
  font-weight: bold;
  display: flex;
  justify-content: flex-end;
}

#add-book {
  float: left;
}

.btn-primary {
  margin-left: 67%;
}

h3 {
  margin-left: 44%;
  margin-bottom: 3%;
}

#contact-section h2 {
  text-align: center;
  margin-bottom: 4%;
}

#contact-section p {
  margin-left: 33%;
}

.contacts {
  margin-left: 36%;
}

.date-book-section {
  display: none;
}