Css background video

Question

Could someone please center the background of this video?

I tried:

margin: 0 auto;
text-align: center;

So far but none of these worked.

This is my code.

Html:

<video autoplay loop poster="polina.jpg" id="vid">
<source src="http://clips.vorwaerts-gmbh.de/big_buck_bunny.webm" type="video/webm">
<source src="http://clips.vorwaerts-gmbh.de/big_buck_bunny.mp4" type="video/mp4">
</video>

CSS:

video#vid {
 position: fixed; right: 0; bottom: 0;
 min-width: 100%; min-height: 100%;
 width: auto; height: auto; z-index: -100;
 background: url(polina.jpg) no-repeat;
 background-size: cover;
 margin: 0 auto;
}

How can I center this video backdrop so that when the window is resized, the left and right sides are removed by the same amount? I appreciate your assistance.