It works for most applications, but Clementine and Keepassxc stay completely “white” and Smplayer has “white” playlist.
In qt5ct I have set the theme as gtk2, and in Kvantum I have tried different themes: default, KvAdaptaDark, and Matcha-Dark/Matcha-Dark-Azul. In all the cases the aforementioned apps stay “white”.
Thanks! The advice on setting QT_QPA_PLATFORMTHEME="qt5ct" in the /etc/environment file worked.
After I made this post I found out something I didn’t pay attention before — those Qt apps that stayed “white” were the ones that I launched via rofi (a dmenu-like launcher) that in its turn was launched by a hot key from sxhkd (a hotkey manager); while the Qt apps affected by the dark theme were ones that I launched in any other way (it so happens that I usually launch some apps one or the other way). But if I launched the “problem” apps from, for example, whisker-menu, they were themed! So the problem was not in the aforementioned apps themselves, but in the fact that the apps launched via rofi were executed in a context where the variable QT_QPA_PLATFORMTHEME was not set. Probably, because rofi was launched by a hot key from sxhkd, which I load at the system start as a user systemd service?
Anyway, after I set this variable in /etc/environment, the theme got applied to all Qt apps regardless of how they are launched.