You'll need to look in your stylesheet to move the Logo to the center. Open: includes/templates/bookshelf/css/stylesheet.css , and find this block of code:

Code:
#logo {
    float: left;
    padding-top: 17px;
    }
and take out the "float: left;" and add "text-align: center;" to make that whole block look like this:

Code:
#logo {
    text-align: center;
    padding-top: 17px;
    }
...........then your logo will be in the center of the header.

Hope this helps.