Great! It’s working now with the following steps in dom0 terminal:
Download qt5-qtstyleplugins RPM file from:
https://kojipkgs.fedoraproject.org//packages/qt5-qtstyleplugins/5.0.0/39.fc32/x86_64/qt5-qtstyleplugins-5.0.0-39.fc32.x86_64.rpm
Move it from your AppVM to dom0 with:
qvm-run --pass-io <from_AppVM> ‘cat /home/user/Download/qt5-qtstyleplugins-5.0.0-39.fc32.x86_64.rpm’ > /home/<your_dom0_user>/Downloads/qt5-qtstyleplugins-5.0.0-39.fc32.x86_64.rpm
Install the package with:
sudo dnf install -y qt5-qtstyleplugins-5.0.0-39.fc32.x86_64.rpm
then insert in /etc/environment:
-------------[Start]---------------------
QT_QPA_PLATFORMTHEME=gtk2
-------------[End]-----------------------
finally you can check, if all changes were set & working:
[TheGardner@dom0]$ cat /etc/environment
QT_QPA_PLATFORMTHEME=gtk2
[TheGardner@dom0]$ echo $QT_QPA_PLATFORMTHEME
gtk2
[TheGardner@dom0]$ sudo dnf info qt5-qtstyleplugins
Qubes OS Repository for Dom0 1.9 MB/s | 3.0 kB 00:00
Installed Packages
Name : qt5-qtstyleplugins
Version : 5.0.0
Release : 39.fc32
Architecture : x86_64
Size : 1.2 M
Source : qt5-qtstyleplugins-5.0.0-39.fc32.src.rpm
Repository : @System
From repo : qubes-dom0-cached
Summary : Classic Qt widget styles
URL : https://github.com/qtproject/qtstyleplugins
License : LGPLv2 or GPLv2
Description : Classic Qt widget styles, including cleanlooks, motif, plastique, qgtk.
FINISHED!
Just restart your machine and voila → Qubes Manager in dark style!