QT applications does not start in dark mode when lauched from .config/autostart in Gnome

I don’t know if the OP still has this issue, but for KeepassXC you can manually change the theme to Dark in KeepassXC -> View -> Theme -> Dark. You can also set it to Classic (Platform-Native) if you have a theme installed and you want it to follow that.

Edit: Probably unrelated
Another thing you can try is use another qt.platformTheme. I use qtct with home-manager and it works without issues. You just need to set the correct theme and icons in qt5ct and qt6ct.

You can have a look at this post which might help you in the right direction: