问如何使用<button>在两个CSS文档之间切换

EN

  <button onclick=getTheme() id=themeToggle>Click to use this theme.</button>
    <select id="select">
        <option>Dark Theme</option>
        <option>Revert To Original</option>
    </select>

function getTheme () {
    function changeTheme (Theme) {
        document.getElementById('style').setAttribute('href', Theme);
    }
    var index = document.getElementById("select").selectedIndex;
    switch (index) {
        case 0:
          changeTheme('css/dark.css');
          break;
          case 1: changeTheme('css/main.css');
    }
}