Rated 5 out of 5 stars

Nice, however, I find the font type used not that good:
there is only a slight difference between bold (unread) messages and the other. Therefore it makes it less convenient to use on a daily basis. There could be simply:
1. the option to change the font type, or make 'unread' x-bold;
2. or a chance to tweak it on user's own. Could the author give the hint where exactly, in which file is the font type declared?
It is the only reason I give 4*, otherwise it is a very nice piece of work, thank you for that.
Edit: man, I am giving 5*, thank you! :))) I was blind not to look at skin folder. :) Now it is extremely customizable. :)

This review is for a previous version of the add-on (3.1). 

Hi Pref, feel free to open an issue on GitHub every time that you want some additional customization (https://github.com/conema/monterail-fulldark/issues).
You can tweak the font by editing the file "chrome/MonterailOverlay/css/root.css" (from row 5 to 8).