style.css

@import url("https://fonts.googleapis.com/css2?family=Roboto:wght@300;400;500;700&display=swap");

* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

body {
  font-family: "Roboto", sans-serif;
  font-weight: 400;
  line-height: 1;
  color: #444;
}

.header {
  background-color: #f03e3e;
  padding: 3rem 4rem;
  color: #fff;
  text-align: center;

  /* make the list items unselectable */
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

.heading-primary {
  font-size: 4rem;
  font-weight: 700;
  margin-bottom: 3rem;
}

.input-field {
  border: none;
  border-radius: 5px;
  width: 75%;
  padding: 1rem;
  font-size: 1rem;
}

.btn-add {
  padding: 1rem;
  width: 25%;
  background-color: #d9d9d9;
  color: #555;
  text-align: center;
  font-size: 1rem;
  cursor: pointer;
  transition: 0.3s;
  border-radius: 5px;
}

.btn-add:hover {
  background-color: #bbb;
}

.main-list li {
  cursor: pointer;
  position: relative;
  padding: 1rem 0.5rem 1rem 4rem;
  background-color: #eee;
  font-size: 1.2rem;
  transition: 0.2s;
}

.main-list li:nth-child(odd) {
  background-color: #f9f9f9;
}

.main-list li:hover {
  background-color: #ddd;
}

.main-list li.checked {
  background-color: #888;
  color: #fff;
  text-decoration: line-through;
}

.main-list li.checked::before {
  content: "";
  position: absolute;
  border-color: #fff;
  border-style: solid;
  border-width: 0 2px 2px 0;
  top: 10px;
  left: 16px;
  transform: rotate(45deg);
  height: 15px;
  width: 7px;
}

.close {
  position: absolute;
  right: 0;
  top: 0;
  padding: 12px 16px 12px 16px;
}

.close:hover {
  background-color: #f44336;
  color: white;
}