I followed the instructions but on the second stage, I got:

root@devuan:/home/astheroth# mv /etc/xdg/Trolltech.conf /etc/xdg/bak.Trolltech.conf
mv: it couldn't be done `stat' over '/etc/xdg/Trolltech.conf': the file or directory doesn't exist.

root@devuan:/home/astheroth# mv /etc/gtk-3.0/settings.ini /etc/gtk-3.0/bak.settings.ini
mv: it couldn't be done `stat' over '/etc/gtk-3.0/settings.ini': the file or directory doesn't exist

Any idea?

That's mine the file is not present, so you don't need to rename/move the original (of course is not present).
Just continue with the other steps.

Best regards

root@devuan:/home/astheroth# mv /etc/xdg/Trolltech.conf /etc/xdg/bak.Trolltech.conf
mv: it couldn't be done `stat' over '/etc/xdg/Trolltech.conf': the file or directory doesn't exist.

root@devuan:/home/astheroth# mv /etc/gtk-3.0/settings.ini /etc/gtk-3.0/bak.settings.ini
mv: it couldn't be done `stat' over '/etc/gtk-3.0/settings.ini': the file or directory doesn't exist

Any idea?

Since KDE-5/Plasma-5 is very difficult to get a homogeneous desktop appearances, for me as lover of 'breeze-dark' is much more difficult. After long recherches I found the solution and don't want to deprive you.
Under KDE-4 was 'qt4config' app (or something similar) , under KDE-5 I think the name was 'qt5ct' but to be installed over a PPA.
Since we cannot use PPAs... we need another solution also practicable for newbie like myself.

1. Tell DE (KDE-5/Plasma-5) to use always is own session. That was reported as bug for 'kdesudo' https://bugs.kde.org/show_bug.cgi?id=353263 but is necessary also for the appearances especially if you want 'Gtk'-apps have exactly same appearances like the 'Qt's, don't ask me why but without this you can stop go further. If not...:
1.a. Create a text-file e.g with Krusader as admin or over konsole

/etc/sudoers.d/workaround_kde5

1.b. Open the new created file, insert following text/code, one emty line @ end, save and close.
Note: by creating file with 'krusader' the editor will opened automatically!

Defaults env_keep += "KDE_SESSION_VERSION KDE_FULL_SESSION"

2. Rename following two files preserving you to change the configuration @ your intent

mv /etc/xdg/Trolltech.conf /etc/xdg/bak.Trolltech.conf

mv /etc/gtk-3.0/settings.ini /etc/gtk-3.0/bak.settings.ini

3. Create SymLinks to the modifiable configurations-files

ln -s ~/.gtkrc-2.0 /etc/gtk-2.0/gtkrc

ln -s ~/.config/Trolltech.conf /etc/xdg/Trolltech.conf

ln -s ~/.config/gtk-3.0/settings.ini /etc/gtk-3.0/settings.ini

4. Logout and login again or restart the system, finish. Enjoy

From now on you will get the same Colors, Icons, Fonts, etc. configured through 'systemsettings5' and Systemsettings as admin/root. Start 'systemsettings' over 'KickOff' or 'Alt+F2' and insert:

systemsettings5

Start 'systemsettings' as admin/root through 'konsole' with:

sudo -i systemsettings5

Note: A restart of system or logout/in is not necessary, sometime you are recommended to restart the actual running apps

Test: Start both 'Qt' and 'Gtk' (Dolphin, Kate, Krusader, Gimp, etc) apps to see the results even with admin-rights like 'Synaptic' over 'KickOff' or:

sudo -i krusader

Note: Some apps like 'Kate' need 'appearances' to be configured in the app same or sometime 'synaptic' don't overtake the Fonts properly. Those issues are very old and as said can be changed in the app same.

Enjoy & best regards.