index.css

.child-container * {
    display: block;
}

.strike {
    text-decoration: line-through;
}

.color-display {
    width: 200px;
    height: 50px;
    border: #555 solid;
    margin: 2px;
}

/* Include the padding and border in an element's total width and height */
.todoapp * {
  box-sizing: border-box;
}

/* Remove margins and padding from the list */
.todoapp ul {
  margin: 0;
  padding: 0;
  list-style-type: none;
}

/* Style the list items */
.todoapp ul li {
  cursor: pointer;
  position: relative;
  padding: 12px 8px 12px 40px;
  background: #eee;
  font-size: 18px;
  transition: 0.2s;
  
  /* make the list items unselectable */
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

/* Set all odd list items to a different color (zebra-stripes) */
.todoapp ul li:nth-child(odd) {
  background: #f9f9f9;
}

/* Darker background-color on hover */
.todoapp ul li:hover {
  background: #ddd;
}

/* When clicked on, add a background color and strike out text */
.todoapp ul li.checked {
  background: #888;
  color: #fff;
  text-decoration: line-through;
}

/* Add a "checked" mark when clicked on */
.todoapp ul 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;
}

/* Style the close button */
.todoapp .closeBtn {
  position: absolute;
  right: 0;
  top: 0;
  padding: 12px 16px 12px 16px;
}

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

/* Style the header */
.todoapp .header {
  background-color: #0c82d1;
  padding: 30px 40px;
  text-align: center;
}

/* Clear floats after the header */
.todoapp .header:after {
  content: "";
  display: table;
  clear: both;
}

/* Style the input */
.todoapp input {
  border: none;
  width: 75%;
  padding: 10px;
  float: left;
  font-size: 16px;
}

/* Style the "Add" button */
.todoapp  .addBtn {
  padding: 10px;
  width: 25%;
  background: #d9d9d9;
  color: #555;
  float: left;
  text-align: center;
  font-size: 16px;
  cursor: pointer;
  transition: 0.3s;
}

.todoapp .addBtn:hover {
  background-color: #bbb;
}


@media (min-width: 1400px) {
  .container {
    width: 1370px;
  }
}
@media (min-width: 1600px) {
  .container {
    width: 1570px;
  }
}

@media (min-width: 1800px) {
  .container {
    width: 1770px;
  }
}