(This is the theme previously known as "Adwaita".)

An easy-to-install version of this theme is available at: https://addons.mozilla.org/en-US/thunderbird/addon/adwaita-thunderbird/

For current issues see: https://github.com/gnome-integration-team/thunderbird-gnome/issues

Help & feedback are both appreciated! ;-)