You actually found it 
There was no sass files for the Shell theme, and it was directly shipped in gnome-shell-common.
You can find the bionic version in ubuntu.css « ubuntu-session-mods « debian - ~ubuntu-desktop/ubuntu/+source/gnome-shell - [no description].
I think you should ship that as a new, separated debian package and hack up the Shell to match it (or ship some new session files + a separate mod)