在 javascript 上移出模态窗口的背景

var modal = document.getElementById('myModal');
var btn = document.getElementById("myBtn");
var span = document.getElementsByClassName("close")[0];

btn.onclick = function() {
  modal.style.display = "block";
}

span.onclick = function() {
  modal.style.display = "none";
}

window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.display = "none";
  }
}

@import url('https://fonts.googleapis.com/css2?family=Roboto:wght@100;400;500;700&display=swap');
@import url('https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css');
body {
  font-family: 'Roboto', sans-serif;
}

h1 {
  margin: 0;
}

p {
  padding-left: 20px;
  padding-right: 20px;
  font-weight: 400;
  font-size: 21px;
}

.fa-plus {
  margin-right: 11px;
}

#myBtn {
  font-size: 16px;
  font-weigth: 600;
  color: #fff;
  border: 0 solid;
  border-radius: 20px;
  padding: 12px 30px;
  background-color: #000;
  text-align: center;
  margin-left: 20px;
  margin-top: 20px;
}

button:focus {
  outline: none;
}

#myBtn:hover {
  background-color: #3C3C3C;
  transition: .2s;
}

button:active {
  background-color: #333;
  border-color: #333;
  color: #eee;
  box-shadow: 0 0 0 3px gray;
}

.modal {
  display: none;
  position: relative;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100px;
  background-color: rgba(90, 90, 114, 0.5);
}

.modal-content {
  background-color: #fff;
  margin: 15% auto;
  padding-top: 20px;
  padding-bottom: 20px;
  width: 50%;
  border-radius: 30px;
  text-align: center;
  font-weight: regular;
}

.modal-title {
  font-size: 21px;
}

.close {
  display: flex;
  justify-content: center;
  align-items: center;
  margin-left: 20px;
  margin-right: -20px;
  float: left;
  font-size: 21px;
  font-weight: 400;
  cursor: pointer;
  color: #000;
  width: 22px;
  height: 22px;
  border-radius: 50%;
  background-color: #FFAAAA;
}

<button id="myBtn">Создать событие</button>

<div class="modal" id="myModal">
  <div class="modal-content">
    <span class="close">&times;</span>
    <h1 class="modal-title">Создание события
      <h1/>
      <hr>
      <p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Nihil consequuntur asperiores ut! Tempore excepturi voluptatibus perferendis vel, repellendus provident adipisci odio distinctio, pariatur unde hic odit magni, nam officia ipsa!</p>
  </div>
</div>