bug#47612: dino and gajim ignore my gtk theme

[bdju]

guix (GNU Guix) 04ba7a196ab23539f1c7897ca5349fa635c5e88d
I am using Guix System.
I have these settings in my ~/.config/gtk-3.0/settings.ini file.
```
gtk-application-prefer-dark-theme = true
gtk-theme-name=Adwaita-dark
```

If I launch either dino or gajim, they have a bright theme.
If I launch them with `GTK_THEME=Adwaita:dark` before the program name,
they appear dark.
pcmanfm successfully has a dark gtk theme. I used lxappearance to set my
gtk theme, but it's just editing the usual files for me.