On GNOME, Unity, Cinnamon, MATE, Xfce, LXDE and all other GTK+ based desktop environments choose a deep-dark theme such as Dorian. If you already run GTK+ 3.20 check out Cloak 3.20 or the darkest variant of Candra 3.20.
These are full-featured themes by killhellokitty that include modifications for Google Chrome (Chromium), Firefox and Thunderbird.