We should not invoke the Jekyll behaviour unless the plantuml configuration contains theme.
We may circumvent most of the Jekyll integration by resolving theme.directory to site.source instead of site.dest, since we then shouldn't need to wait until Jekyll has written to site.dest before invoking PlantUML.
As I write in https://github.com/SwedbankPay/kramdown-plantuml/issues/84#issuecomment-974011968, we should make the Jekyll integration less obtrusive:
plantumlconfiguration containstheme.theme.directorytosite.sourceinstead ofsite.dest, since we then shouldn't need to wait until Jekyll has written tosite.destbefore invoking PlantUML.