How to resolve relative paths in css with Brunch?

Question

For example, I use 3rd party library that has style definition like background: url(img/blue.png) in it's css file. When Brunch concatenates all files together and puts output file to public folder, then this URL doesn't work anymore because it still has relative path. And browser gets 404 error while trying to load this image. Is there any plugin or something like that to replace relative paths in stylesheet's url()/import() directives automatically?