If I have understood it correctly from Aravisian's advice, do the following:
Elevate to root with
pkexec thunar
and enter your password. This will open your filemanager with sudo rights. I changed the command, because in Zorin lite thunar is the default file manager.
Then go to
/etc/X11/Xsession.d/56-zorin-os-lite-session,
right click on it >open with texteditor
and change 'gtk2' into 'qt5ct' in line
export QT_QPA_PLATFORMTHEME=qt5ct
Save the file.
Logout and login again.
