// 获取下拉选项的DOM元素
var select = document.getElementById("mySelect");

// 监听下拉选项的改变事件
select.addEventListener("change", function() {
  // 获取当前选中选项的值
  var selectedOption = select.options[select.selectedIndex].value;
  
  // 根据选项值修改CSS样式
  if (selectedOption === "option1") {
    select.style.backgroundColor = "red";
  } else if (selectedOption === "option2") {
    select.style.backgroundColor = "blue";
  } else {
    select.style.backgroundColor = "green";
  }
});