Rated 5 out of 5 stars

Very pretty theme.

Also I recommend userstyle for full darkness!
https://userstyles.org/styles/65016/tt-deepdark-colored-mail

For quick switch between userstyle on/off I use keyconfig and such rules:
Alt+S (switch on) Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefBranch).setBoolPref("extensions.stylish.styleRegistrationEnabled", false);
Alt+D (switch off) Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefBranch).setBoolPref("extensions.stylish.styleRegistrationEnabled", true);

This review is for a previous version of the add-on (11.0.2).