In my original post, the command
gsettings set org.gnome.desktop.interface gtk-theme "Adwaita-dark"
was sufficient to apply a dark colour scheme to pretty much everything.
But since then, certain apps, including Calls, Chatty and Contacts, have been updated and the gtk-theme setting alone no longer darkens them.
It is necessary to also indicate a dark colour scheme preference:
gsettings set org.gnome.desktop.interface color-scheme "prefer-dark"
Then everything is dark again.
I haven’t tried installing Gnome Tweaks again. When I mark it for installation in aptitude, the relationship to gnome-shell is no longer merely “recommends” and it now insists on pulling in gnome-shell and gdm3. Makes sense, I suppose; it is Gnome Tweaks, after all.
Based on @amarok’s experience, I’d guess that, if I proceed with the installation, it won’t set gdm3 as the default display manager and turn my phone into a desktop. But it just seems messy to me when I can run a couple of gsettings commands instead.
My intended meaning was something like “I anticipate that some people might be disappointed that there isn’t a convenient GUI way to do this, and I suppose they might have a point, but personally I’m contented with this method. (Sorry folks.)”