I am not testing nor using your work for the time being, just following the discussions
here. I send this mail to you because an idea just popped up while reading your answer,
so I'll submit it to you under the shape of a question: wouldn't it be a good idea if you
managed to introduce a fallback into your code? ie: if no icon theme is defined by the
user, having hicolor icon theme as a fallback, as it is a very generic theme which is
usually present in all distributions?
libqtxdg does that at an icon level. This discussion started with a bug in libqtxdg. But the issue is not in libqtxdg.
Here, we are discussing it at an theme level. lxqt-qtplugin has an hicolor fallback, but due to the way lxqt-qtplugin and liblxqt interact, it's not effective.