Bug#870034: blackbird-gtk-theme: Consider adding a dependency on gtk3-engines-xfce
unlike some other themes, the Blackbird theme actually works fine with recent GTK versions (at least in my experience). However, on XFCE the theming only seems to be enabled for certain applications (e.g. Thunderbird) when gtk3-engines-xfce is installed.
Please consider adding gtk3-engines-xfce as a dependency so that this theme works out of the box for all applications.