Vertically align the modal dialog close buttons within the header.

Before: 
![before](http://f.cl.ly/items/1B0i2P051e2r3f2J0C2k/Screen%20Shot%202013-05-21%20at%2012.38.10%20PM.png "before")

After:
![after](http://f.cl.ly/items/0Q2X0o3e2M291g0u3E0m/Screen%20Shot%202013-05-21%20at%2012.53.20%20PM.png "after")

(The `line-height` of the `<h1>` in the header is set to 40 pixels, and there is a default non-zero `top-margin` defined elsewhere.)