As much as I hate replying to myself and spamming this list, some might
actually prefer a black-and-white version of a notmuch icon based on the
scales of justice :-). Sorry, I won't post about this anymore.

http://github.com/spaetz/notmuch-all-feature/raw/master/contrib/notmuchscales_alt.png

(svg here:
http://github.com/spaetz/notmuch-all-feature/tree/master/contrib/)