I should say that I am aware of the workaround, e.g.

    (setq
      notmuch-search-line-faces '(("unread" :weight bold)
                                  ("flagged" :inherit 'font-lock-string-face)))

but this must be applied per-user. If you removed this defcustom and had
unread/flagged/other faces, then themes (such as darcula) would be able
to provide sensible defaults out of the box.