emacs: make faces readable on dark backgrounds.

The notmuch-tag-flagged, notmuch-search-flagged-face and
notmuch-crypto-part-header faces defaulted to "blue", which is nearly
unreadable when a dark background is in use.  This is addressed by using
"LightBlue1" for dark backgrounds.

As a side effect, these faces are now no-op definitions for grayscale or
mono displays.

emacs/notmuch-crypto.el

@@ -42,7 +42,12 @@ mode."
   :group 'notmuch-crypto)
 
 (defface notmuch-crypto-part-header
-  '((t (:foreground "blue")))
+  '((((class color)
+      (background dark))
+     (:foreground "LightBlue1"))
+    (((class color)
+      (background light))
+     (:foreground "blue")))
   "Face used for crypto parts headers."
   :group 'notmuch-crypto
   :group 'notmuch-faces)

emacs/notmuch-tag.el

@@ -115,7 +115,12 @@ Used in the default value of `notmuch-tag-formats`."
   :group 'notmuch-faces)
 
 (defface notmuch-tag-flagged
-  '((t :foreground "blue"))
+  '((((class color)
+      (background dark))
+     (:foreground "LightBlue1"))
+    (((class color)
+      (background light))
+     (:foreground "blue")))
   "Face used for the flagged tag.
 
 Used in the default value of `notmuch-tag-formats`."

emacs/notmuch.el

@@ -313,7 +313,11 @@ there will be called at other points of notmuch execution."
   :group 'notmuch-faces)
 
 (defface notmuch-search-flagged-face
-  '((t
+  '((((class color)
+      (background dark))
+     (:foreground "LightBlue1"))
+    (((class color)
+      (background light))
      (:foreground "blue")))
   "Face used in search mode face for flagged threads.