Remove superfluous background-color property in definition of ewf-picker class

Addresses issue #111. With the patch it looks like this: 

![111](http://f.cl.ly/items/002b1U3l1F0w3q1y0G1U/Screen%20Shot%202013-05-20%20at%2010.54.27%20AM.png "111")

(We may need to generally rethink the colors in this dialog, in particular the classification buttons on the left.)