move fill color for icons to CSS

reworked icon fills to have easily modify-able background color:

![screen shot 2014-04-15 at 2 26 16 pm](https://cloud.githubusercontent.com/assets/108094/2711227/8e502040-c4cb-11e3-83a0-ff73bc0aa76b.png)

@jfirebaugh do you want to have a look at this and let me know what you think?