This mixin will be on jsf side

resource-mixins.scss

//-----------------------------------------------------------------------------
// resource_url function returns the parameter as url(#{resource['<parameter>']})
// and should be used instead of CSS url() or compass image_url() in JSF applications.
// Define JSF Resource Library resource['standard:

@function resource_url($url) {

   @return ("\#{resource['#{$url}']}");
}