Hi Wolfgang, the problem is well known and your one-liner is not part of the solution. a) there should be something depend on desktop-base b) frost at all don't look well with debian - a debian theme is needed c) i guess it was in 0.13 that we (i) moved all upstream configurations to /usr/share/$foo - the reason was to have /etc/xdg/foo and /etc available for distributions - so there is no need to touch pcmanfm-qt at all.
Possible solution: Be our guest and provide your wished theme to https://salsa.debian.org/lxqt-team/lxqt-themes-extra - i would suggest a theme for debian-edu. The "only" thing we need after finishing is a DD who want to sponsor the new package. Cheers Alf