动态在标头中创建站点顶部通知栏

Is there a way to insert a notification bar at the top of any webpage (similar to the ones here at Stack Overflow) without needing any special code in the page itself? I'm looking for either a snippet of CSS styling or, if necessary, a Javascript example. It doesn't need any special animations or even a close link, but it does need to make room for itself at the top of the page. Is there a way to insert a notification bar at