Added documentation for this at https://extensions.xwiki.org/xwiki/bin/view/Extension/Flamingo%20Theme%20Application?viewer=changes&rev1=139.12&rev2=140.1&
https://extensions.xwiki.org/xwiki/bin/view/Extension/Flamingo%20Theme%20Application#HExamples
Closing as a `Won't Fix`, because I don't believe we want to support incomplete syntax like the one described in the ticket.