In CSS I am trying to use this to access an image relative to the classpath root:

-fx-background-image: url("/mint/css/report_16x16.png");

That does not work, the image is not to be seen. It does work though when declaring the style like this:

-fx-background-image: url("report_16x16.png");

Actually, the CSS documentation does not seem to allow the former use but according to RT-21967 it should be available, too (since Lombard). See code example in first comment.