Board Thread:Support Requests - Getting Technical/@comment-25916964-20150829122556/@comment-4731443-20150830035158

Headers are meant to be used in groups. If the group is empty, then the header won't show - no modification required.

Also, according to Help:PortableInfoboxes/Tags, theme is not a valid parameter for data, and source is not a valid parameter for header, so I very much doubt the validity of the above code. I recommend reading though Help:PortableInfoboxes for how groups and headers behave.