Darkened Color of Sort Icons

Before (14px)
![screen shot 2014-09-25 at 6 18 12 pm](https://cloud.githubusercontent.com/assets/910510/4413240/2fb28d2c-4502-11e4-997c-3b4b7a693da5.png)

After (18px)
![screen shot 2014-09-25 at 6 17 45 pm](https://cloud.githubusercontent.com/assets/910510/4413238/2c9be296-4502-11e4-80a3-b0634f339fc0.png)