There are a couple of ways by which external stylesheets may be linked to an SVG document. I know of the following three:

• The W3C's formal method for doing so, by use of a <?xml-stylesheet ?> tag. Details here.
• The use an @import CSS directive inside an inline stylesheet.

This package currently does not support either method.

I see that some effort has already been made to fix this, and also that this package is no longer maintained. But I figured to create this issue for the benefit of other people who may be interested in this feature.