index.css

        *, *:before, *:after {
    box-sizing: border-box;
  }

  body {
    background: #fff;
    font-family: arial;
    padding:50px;
  }
  h1{
  text-align: center;
  }

  .container{
  width:1100px;
  margin: 0 auto;
  }

  .modal-container .modal-btn {
    display: block;
    color: #fff;
    background: #e74c3c;
    width: 160px;
    height: 50px;
    font-size: 22px;
    line-height: 50px;
    text-align: center;
    border-radius: 3px;
    cursor: pointer;
    box-shadow: 0 5px 5px -5px #333;
    transition: background 0.3s ease-in;
  }
  .modal-container .modal-btn:hover {
    background: #c0392b;
  }
  .modal-container .modal-content,
  .modal-container .modal-backdrop {
    height: 0;
    width: 0;
    opacity: 0;
    visibility: hidden;
    overflow: hidden;
    cursor: pointer;
    transition: opacity 0.2s ease-in;
  }
  .modal-content h2{
  border-bottom: 2px solid #000;
  padding-bottom: 5px;
  }
  .modal-container .modal-close {
    color: #aaa;
    background: #fff;
    width: 25px;
    height: 25px;
    padding-top: 3px;
    font-size: 26px;
    font-weight: bold;
    text-align: center;
    position: absolute;
    right: 5px;
    top: 5px;
    cursor: pointer;
  }
  .modal-container .modal-close:hover {
    color: #333;
  }
  .modal-container .modal-content-btn {
    position: absolute;
    bottom: 20px;
    right: 30px;
    background: #3498db;
    color: #fff;
    width: 50px;
    height: 32px;
    border-radius: 2px;
    font-size: 14px;
    font-weight: normal;
    text-align: center;
    cursor: pointer;
    padding-top: 9px;
  }
  .modal-container .modal-content-btn:hover {
    color: #fff;
    background: #2980b9;
  }
  .modal-container #modal-toggle {
    display: none;
  }

  .modal-container #modal-toggle:checked ~ .modal-backdrop {
    background-color: rgba(0, 0, 0, 0.6);
    width: 100vw;
    height: 100vh;
    position: fixed;
    left: 0;
    top: 0;
    z-index: 9;
    visibility: visible;
    opacity: 1;
    transition: opacity 0.2s ease-in;
  }
  .modal-container #modal-toggle:checked ~ .modal-content {
    background-color: #fff;
    max-width: 400px;
    width: 400px;
    height: 300px;
    padding: 10px 30px;
    border-radius: 4px;
    position: fixed;
    left: calc(50% - 200px);
    top: 12%;
    z-index: 999;
    cursor: auto;
    visibility: visible;
    opacity: 1;
    box-shadow: 0 3px 7px rgba(0, 0, 0, 0.6);
  }
  @media (max-width: 400px) {
    .modal-container #modal-toggle.active ~ .modal-content, 
    .modal-container #modal-toggle:checked ~ .modal-content {
      left: 0;
    }
  }