Wrappers and #divs are not centering

Hi,
I wanted to create a design with centered header + menu, and under that a main content column on the left and a sidebar on the right.

Tried to center the zones and their belonging regions with a width and margin in the global.css:

      #zone-menu-wrapper {
    background-color: transparent;
    width: 990px;
    margin-left: auto;
    margin-right: auto;
    }
        #region-menu {
            background-color: #000044;
            width: 990px;
            margin-left: auto;
            margin-right: auto;
            }

Normally, everything should be centered, but now its about 15 px more to the right and on the left is a small space.

What did I wrong?

Thank you in advance!