CSS水平居中的固定div？

#menu {
    position: fixed;
    width: 800px;
    background: rgb(255, 255, 255); /* The Fallback */
    background: rgba(255, 255, 255, 0.8);
    margin-top: 30px;
}

我知道这个问题是一百万次，但我不能找到一个解决我的情况。
我有一个div，应该固定在屏幕上，即使页面被滚动，它应该总是保持CENTERED在屏幕中间！

I know this question is a million times out there, however I can't find a solution to my case. I've got a div, which should be fixed on the screen, even if the page is scrolled it should always stay CENTERED in the middle of the screen!

div应该有 500px width，应该距离顶部（margin-top） 30px

The div should have 500px width, should be 30px away from the top (margin-top), should be horizontally centered in the middle of the page for all browser sizes and should not move when scrolling the rest of the page.

这是否可能？