[Plugin: NextGEN Gallery] CSS Validation

When I try to validate CSS, there is this problem
URI : http://www.toniocasado.com/wp-content/plugins/nextgen-gallery/css/ngg_dkret3.css?ver=1.0.0
72 .ngg-album-compactbox Value Error : background Parse Error [empty string]

This is the property:
.ngg-album-compactbox {
background:transparent url(albumset.gif) no-repeat scroll 0%;
}

How can we modify this CSS?

http://wordpress.org/extend/plugins/nextgen-gallery/