[PATCH v3] emacs: make faces readable on dark backgrounds.

[Matt Armstrong]

This is a followup to
id:1476987754-13672-1-git-send-email-marmstrong at google.com with:

 - Addition of ((class color) (background light)) tests for the light
   faces, as requested by Mark.  This is consistent with how many
   other light/dark faces are already defined in notmuch.

 - Switch to a blue color, as opposed to "gold".  This makes sense,
   because light backgrond face uses "blue".  David suggested "deep
   sky blue" but I found that pretty fruit-salady and actually darker
   than the normal text.  This tended to deemphasize flagged email
   when mixed among non-flagged mail in notmuch-show mode.  The point
   is the opposite.  I settled on LightBlue1, which becomes cyan on 8
   color displays (xterm), while likely to be subtly different from
   whatever default text color the user has set up.

 - renamed patch from "emacs: make faces clear on dark backgrounds"
   (s/clear/readable/).