Qt apps (keepassxc, pinentry-qt) rendered light while the GTK side was
Adwaita-dark. keepassxc's `auto` theme reads dark/light only from the
xdg-desktop-portal `color-scheme` value, which the gtk portal serves from
dconf `org/gnome/desktop/interface/color-scheme` — unset, so it reported
"no preference" and fell back to light. Set it to `prefer-dark`.
Also route Qt through the `gtk3` platform theme (`libqgtk3` ships in
qtbase, unlike an external `adwaita-qt` style plugin which nixpkgs' baked
`QT_PLUGIN_PATH` makes unreachable), and set the GTK prefer-dark flag.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>