js中alert的样式修改

alert('这是一条消息');

<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close-button">&times;</span>
    <p>这是一条消息</p>
  </div>
</div>

.modal {
  display: none;
  position: fixed;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0,0,0,0.4);
}

.modal-content {
  background-color: #fefefe;
  margin: 15% auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}

.close-button {
  color: #aaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
}

.close-button:hover,
.close-button:focus {
  color: black;
  text-decoration: none;
  cursor: pointer;
}

// 获取模态框元素
var modal = document.getElementById("myModal");

// 获取打开模态框的按钮
var btn = document.getElementById("openModalBtn");

// 获取关闭模态框的按钮
var span = document.getElementsByClassName("close-button")[0];

// 点击按钮打开模态框
btn.onclick = function() {
  modal.style.display = "block";
}

// 点击关闭按钮关闭模态框
span.onclick = function() {
  modal.style.display = "none";
}

// 点击窗口其他区域关闭模态框
window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.display = "none";
  }
}