Updated to be accurate to current Zorin OS:
In terminal, download the qt5 plugins and qt5ct configuration tool:
sudo apt install qt5ct qt5-style-plugins
Once installed, you will need to change some things in Root:
sudo -i
nautilus
or
thunar
Or...
nemo
navigate to /etc/profile.d
Open qt-style-override.sh and remove the line there, blanking the file. Save and close.
next, open `qt-qpa-platformtheme.sh and change its contents to:
export QT_QPA_PLATFORMTHEME=qt5ct
Close out the Elevated File manager.
Open the app menu and launch qt5ct
You can select any of the built in themes or use your system theme by selecting gtk in each drop down menu near the top.
To enable Qt6 support, the process is similar to the above.
sudo apt install qt6ct qt6-gtk-platformtheme
In the same process as the above, instead use:
export QT_QPA_PLATFORMTHEME=qt6ct