COSMIC desktop - dark/light mode not detected

[digitaltrails]

When running in the COSMIC desktop, Qt applications don't pick up the light/dark style of the desktop. Also, not Qt event is received when the desktop changes from light to dark.

This appears to be a problem with COSMIC's support of Qt - see pop-os/cosmic-settings#617. In the referenced issue, setting
QT_QPA_PLATFORMTHEME=xdgdesktopportal is reported to help, but setting that as well as installing libqt5-qtbase-platformtheme-xdgdesktopportal and xdg-desktop-portal-cosmic had no effect.

[digitaltrails]

The following can force a dark look:

% qt5ct    # run the qt5ct GUI and set a darker look
% QT_QPA_PLATFORMTHEME=qt5ct vdu_controls
% vdu_controls --platformtheme qt5ct      

Need to find out more about QT_QPA_PLATFORMTHEME

QPA: Qt Platform Abstraction, see https://wiki.archlinux.org/title/Qt

[digitaltrails]

Try some of the above with Qt6