fix "works offline" icon (bug 1022793)

* fixes broken path to image (must be relative to CSS file when deployed)
* uses `background` shorthand
* uses adjusted spacings

![](https://www.dropbox.com/s/gdgt2zctoff9w5t/Screenshot%202014-09-09%2012.43.54.png?dl=1)
![](https://www.dropbox.com/s/rhpjf4czlcw5b6h/Screenshot%202014-09-09%2012.44.04.png?dl=1)

r? @spasovski 
I know you have a local branch with coffeescript'ed commonplace.