Only the first occurrence of url(x) was being replaced

Where an url(x) is used more than once in a css file, only the first occurrence was actually being replaced.

Example taken from the Colorbox library.
https://github.com/jackmoore/colorbox/blob/master/example1/colorbox.css

```
#cboxMiddleLeft{width:21px; background:url(images/controls.png) left top repeat-y;}
#cboxMiddleRight{width:21px; background:url(images/controls.png) right top repeat-y;}
#cboxTopCenter{height:21px; background:url(images/border.png) 0 0 repeat-x;}
#cboxBottomCenter{height:21px; background:url(images/border.png) 0 -29px repeat-x;}
```