Seems Scribus can handle themes it founds on the system: however I still don't get how exactly it works, I have many themes under /usr/share/themes, but I can choose just few of them from Scribus preferences...
Anyway I tried to add adwaita qt theme:
https://github.com/FedoraQt/adwaita-qt(from here:
https://slackbuilds.org/repository/15.0/desktop/adwaita-qt/?search=adwaita)
And now I can choose also those two themes in light and dark version.
Nice, a part of some issue...
When I set "Content Properties" hooked to the, so not free, Seems all scribus window cannot stay in within screen height. It's a bit strange issue: when I open scribus, its window is all contained right as expected, but if I switch to an other workspace and switch back again to the one of scribus, I notice its window a bit higher than the screen, and I can't see the part at bottom cause it ends out of screen.
I tried also after switching to some other theme and the problem disappears. Obiously it re-pops up if I set back adwait-dark theme.

The issue seems related to the content properties bar I suppose, because even when I unhook it, I cannot reduce its height. If I switch to an other theme I can properly resize it as I want.