CSS：页面顶部的精简横幅（如本页中的橙色横幅）

I need to put a thin banner on the top of the page kinda like the orange notification one on this page. I need it to be absolutely positioned though, because when you click on it it needs to drop down (over the current page, not pushing the current page down). The banner needs to stretch the entire width of window but the content needs to be within 800px in the middle. I have it already made but I am having trouble with the CSS positioning of it.I need to put a thin banner on the top of the p