wget https://github.com/spymastermatt/thunderbird-monterail/archive/master.zip
unzip master.zip
mv thunderbird-monterail-master ~/.thunderbird/[XXXXXXX].default/chrome
In file ~/.thunderbird/[XXXXXXX].default/chrome/userChrome.css replace line containing @import "themes/system.css" with one of the available themes, e.g. "themes/dark.css".
Note: (ls chrome/themes to see full list of themes)
'View' > 'Layout' > select 'Vertical View'