Fix browse dialog button focus

Removes the button focus outline in the Browse dialog, which is broken as described in issue #39, in favor of a subtly darker background color. It looks like this: 

![focus](http://f.cl.ly/items/42183r442J27472c1L0c/Screen%20Shot%202013-05-08%20at%2011.23.58%20AM.png "focus")

(The third "M" button is highlighted above.)