margin-right is not working with my HTML. How can I center my content?

Recently, I am learning Responsive Design.  But I’ve got some trouble in centering my content when the screen gets big.

Here is the problem:

@media screen and (min-width: 950px) {
    body {
        margin: 10%;
    }
}

When I set the margin to 10%, only the margin-top and margin-left work.  This is not what I want.  margin-right doesn’t work at all.

Here is the picture:

So, how can I fix this?