OpenXML4J - Open XML library for Java

I would like to get the style part of the word document.

Package pkg = Package.open(filePath, PackageAccess.READ);

PackageRelationship coreDocumentRelationship = pkg
.getRelationshipsByType(PackageRelationshipTypes.CORE_DOCUMENT)
.getRelationship(0);

PackagePart coreDocumentPart =
pkg.getPart(coreDocumentRelationship);

PackageRelationship styleRelationship =
coreDocumentPart
.getRelationshipsByType(PackageRelationshipTypes.STYLE_PART)
.getRelationship(0);

PackagePart stylePart = pkg.getPart(styleRelationship);

However, stylePart is returned as null. I know I should use PackagingURIHelper, but I don't know how to use it.

Thanks.