I need help with dark mode in 4.2.1

[renehoj]

The same thread explains what you need to do

Dark Theme for [Qubes Manager] Not Working in 4.1

export QT_QPA_PLATFORMTHEME=gtk2 in /etc/environment equally does the trick, if the respective dependencies are installed. Not so important in dom0 but if you want to push it into all your templates, it’s nicer to do it this way instead of having to run qt5ct.