After the update from
we now have Plasma and QT6 as defaults. Of course, some apps are still using qt5, so we now have a “nice” mix of both worlds (as if the different theming for gtk2,3,4,flatpaks,qt5 etc. was not enough 
)
So, the first thing we have to do to restore the look of QT apps after this update is to install kvantum-qt5. This will restore the look for the apps, they will use kvantum and the selected theme, like matcha-dark-sea for example.
The next problem is that many apps, like qbittorrent or ark, are qt6 apps. After the update, QT6 scales automatically for high DPI resolutions…but obviously in a different way than XFCE. So if you have scaled XFCE in the Appearance Settings from the default 96px, to, for example 120p, the QT apps will be uncomfortably big. So to achieve the same level of scaling, we actually have to scale QT down…
this is done with adding the line
export QT_SCALE_FACTOR=0.8
in your .profile (see above!!)
0.8 or 80% is an example.
Unfortunately, QT_SCALE_FACTOR fixes the qt6 app but completely breaks everything qt5.