Fix warnings for helm faces ('nil value is invalid, use ‘unspecified’…

… instead') (bbatsov#382)

zenburn-theme.el

@@ -838,8 +838,8 @@ Also bind `class' to ((class color) (min-colors 89))."
    `(helm-time-zone-current ((t (:foreground ,zenburn-green+2 :background ,zenburn-bg))))
    `(helm-time-zone-home ((t (:foreground ,zenburn-red :background ,zenburn-bg))))
    `(helm-bookmark-addressbook ((t (:foreground ,zenburn-orange :background ,zenburn-bg))))
-   `(helm-bookmark-directory ((t (:foreground nil :background nil :inherit helm-ff-directory))))
-   `(helm-bookmark-file ((t (:foreground nil :background nil :inherit helm-ff-file))))
+   `(helm-bookmark-directory ((t (:foreground unspecified :background unspecified :inherit helm-ff-directory))))
+   `(helm-bookmark-file ((t (:foreground unspecified :background unspecified :inherit helm-ff-file))))
    `(helm-bookmark-gnus ((t (:foreground ,zenburn-magenta :background ,zenburn-bg))))
    `(helm-bookmark-info ((t (:foreground ,zenburn-green+2 :background ,zenburn-bg))))
    `(helm-bookmark-man ((t (:foreground ,zenburn-yellow :background ,zenburn-bg))))
@@ -859,7 +859,7 @@ Also bind `class' to ((class color) (min-colors 89))."
    `(helm-grep-file ((t (:foreground ,zenburn-fg :background ,zenburn-bg))))
    `(helm-grep-finish ((t (:foreground ,zenburn-green+2 :background ,zenburn-bg))))
    `(helm-grep-lineno ((t (:foreground ,zenburn-fg-1 :background ,zenburn-bg))))
-   `(helm-grep-match ((t (:foreground nil :background nil :inherit helm-match))))
+   `(helm-grep-match ((t (:foreground unspecified :background unspecified :inherit helm-match))))
    `(helm-grep-running ((t (:foreground ,zenburn-red :background ,zenburn-bg))))
    `(helm-match ((t (:foreground ,zenburn-orange :background ,zenburn-bg-1 :weight bold))))
    `(helm-moccur-buffer ((t (:foreground ,zenburn-cyan :background ,zenburn-bg))))