如何更改自定义警报对话框的背景

<div id="customAlert" class="alert" style="display:none;">
  <span class="alert-text"></span>
 <button id="alertButton">确定</button>
</div>

.alert {
  background-color: #f2dede;
  border: 1px solid #ebccd1;
  border-radius: 5px;
  color: #a94442;
  display: block;
  margin: 0 auto;
  padding: 10px;
  position: fixed;
  top: 20%;
  left: 50%;
  transform: translate(-50%, -50%);
  width: 300px;
  z-index: 9999;
}

.alert-text {
  display: block;
  margin-bottom: 10px;
}

#alertButton {
  background-color: #d9534f;
  border: none;
  border-radius: 3px;
  color: #fff;
  cursor: pointer;
  display: block;
  margin: 0 auto;
  padding: 5px 10px;
}

function showAlert(message) {
  const alert = document.getElementById('customAlert');
  const alertText = alert.querySelector('.alert-text');
  alertText.textContent = message;
  alert.style.display = 'block';
}

function hideAlert() {
  const alert = document.getElementById('customAlert');
  alert.style.display = 'none';
}

document.getElementById('alertButton').addEventListener('click', hideAlert);