index.css

/* AWESOME BOOKS */

* {
  font-family: 'Comic Sans MS', 'Comic Sans', cursive;
  margin: 0;
  box-sizing: border-box;
}

body {
  display: flex;
  flex-direction: column;
  height: 100vh;
}

#book_list,
#add_books,
#contact-information {
  flex-grow: 3;
}

.navigation-bar,
footer {
  display: flex;
  border: 3px solid black;
  margin: 1rem;
  justify-content: space-between;
  align-items: center;
  padding: 1rem;
}

.navigation-bar ul {
  list-style: none;
  display: flex;
  font-size: 13px;
  padding-left: 0;
}

.navigation-bar ul li a {
  color: #000;
  text-decoration: none;
  border-right: 1px solid black;
  padding: 0 1rem;
}

.navigation-bar ul li:nth-last-child(1) a {
  border-right: none;
  padding-right: 0;
}

form h2 {
  margin-top: 30px;
}

.navigation-bar h2 {
  font-size: 16px;
}

.navigation-bar ul li:nth-last-child(3) a {
  color: rgb(58, 169, 206);
}

.date {
  font-size: 14px;
  text-align: end;
  margin-right: 1rem;
}

h1 {
  text-align: center;
}

#book_collection {
  margin: 10px 10px 50px 10px;
}

#book_list li {
  padding: 10px;
  display: flex;
  justify-content: space-between;
}

#book_list li:nth-child(odd) {
  background-color: #80808057;
}

.contact-information ul {
  margin-top: 2rem;
}

#book_list ul {
  list-style: none;
  padding-left: 0;
  border: 3px solid black;
}

#book_collection button,
#add_button {
  background-color: white;
  border: 2px solid black;
  box-shadow: 3px 2px 1px black;
}

form {
  margin: 10px;
  display: flex;
  flex-direction: column;
  align-items: center;
}

input {
  margin: 10px;
  width: 70%;
  border: 2px solid black;
}

#add_button {
  align-self: flex-end;
  width: auto;
  margin-right: 3.75rem;
}

::placeholder {
  color: black;
  opacity: 1;
}

.contact-information {
  display: flex;
  flex-direction: column;
}

.contact-information h2 {
  margin: 2.5rem 1rem;
  align-self: center;
}

.contact-information p {
  margin: 0 2rem;
}

.collapse {
  display: none;
}

@media screen and (min-width: 700px) {
  #book_collection {
    width: 60%;
    margin: 50px auto;
  }

  hr {
    margin-top: 50px;
  }

  form {
    width: 25%;
    margin: auto;
  }

  #add_button {
    margin-right: 4rem;
  }

  #book_collection button:hover,
  #add_button:hover {
    background-color: #1b1b1b57;
    color: #fff;
  }

  .navigation-bar h2 {
    font-size: 22px;
  }

  .navigation-bar ul {
    font-size: 18px;
  }

  .contact-information {
    align-self: center;
  }
}