Thunderbird is technically rendered with GTK on Linux, but is mostly written using web tech. They are mostly not GTK, I'm don't think GTK is even involved on Wi

Thunderbird is technically rendered with GTK on Linux, but is mostly written using web tech. They are mostly not GTK, I'm don't think GTK is even involved on Windows for instance (don't know about Mac). For this reason, I doubt they will follow everything gtk Gnome does.