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'm just being forward thinking for when we port our Stylus back to vanilla CSS because CSS preprocessors aren't cool anymore.

P.S. CSS preprocessors aren't cool anymore. Just like Node.js isn't cool anymore.