I use a transparent gnome-terminal (lately also gnome-console). With no custom theme the background of emacs behaves exactly like the terminal. If I use tango-dark however (or any other custom theme for that matter) the background transparency is all but gone.
Is there a way to avoid this? I tried to edit the /usr/share/emacs/27.2/etc/themes/tango-dark-theme.el or a local copy of it, but I can't find the line responsible for general background behaviour.