[css] How to center a div with Bootstrap2?

http://twitter.github.com/bootstrap/scaffolding.html

I tried like all combinations:

<div class="row">
  <div class="span7 offset5"> box </div>
</div>

or

<div class="container">
  <div class="row">
    <div class="span7 offset5"> box </div>
  </div>  
</div>

changed span and offset numbers...

But I cant get a simple box perfectly centered on a page :(

I just want a 6-column-wide box centered...

did it with

<div class="container">
  <div class="row" id="login-container">
    <div class="span8 offset2">
       box
    </div>
  </div>
</div>

But the box is too wide, is there any way I can do it with span7 ?

span7 offset2 gives extra padding to the left span7 offset3 extra padding to the right...