Grunt - change asset linking when building for production

I've built a mobile version of an existing website. The site I've built is completely static and relies only on HTML/CSS/JS. This site resides on the same server as the desktop version and is served from under http://example.com/mobile as a subdirectory. As such, I've set the <base> tag to:

<base href="http://example.com/mobile/"> 

The only access I get to the existing site is via FTP, so I don't have too many options to reconfigure the already existing server. And I don't want to either.

My mobile version is only somewhat functioning - all the <script> and <link> tags inside index.html correctly point to http://example.com/mobile/{js,css,img}, while links used inside js or css files look up to the top domain (e.g - http://example.com/img/image.png instead of http://example.com/mobile/img/image.png).

I can manually copy all the assets for the links to work, but since this is a Grunt project I figured there might be a smarter way to do this.

So, how can I have all internal asset links in js or scss files change when building (grunt build)? Something like:

site.css

// in dev body { background: url('/img/image.png') top left no-repeat; } // after building body { background: url('/mobile/img/image.png') top left no-repeat; } 

Thanks.