I may be misunderstanding how the API should behave, but I would expect this query:

https://de.wikivoyage.org/w/api.php?action=parse&format=json&page=Hauptseite&prop=modules%7Cjsconfigvars%7Cheadhtml

To have values in modulestyles that match the link[rel=stylesheet] modules (e.g. mediawiki.skinning.interface, mediawiki.skinning.content.parsoid, etc.)

Instead modulestyles is just []

Original Issue: https://github.com/openzim/mwoffliner/issues/873

Thanks in advance