Thunderbird dark in Ubuntu 16.04 (Xenial Xerus)

[Narcis Garcia]

In Ubuntu 14.04 (Trusty) I installed gnome-themes-extras to get
Darklooks GTK theme. This has been useful to link ~/.gtkrc-2.0 to
theme's file, and get Mozilla software working with same dark colors as
Gnome3 dark desktop.

In Ubuntu 16.04 the package gnome-themes-extras has disappeared (I've
searched Darklooks with apt-file too).
How can I now integrate Mozilla applications to dark color scheme?

Thanks.