Make modal "x" work on light and dark BGs without toggling class.

![screen shot 2014-05-19 at 2 05 06 pm](https://cloud.githubusercontent.com/assets/583202/3018003/b5e41ede-df80-11e3-8d51-322f05b0959f.png)
![screen shot 2014-05-19 at 2 05 23 pm](https://cloud.githubusercontent.com/assets/583202/3018004/b7475fac-df80-11e3-890b-7c084d5062ef.png)

Tried out using a light gray (`#ccc`) rather than toggling between dark gray and white. Seems to work pretty well, and means one less thing we need to specify when creating modals. References DoSomething/dosomething#2159.