add header variable to example config

exampleSite/config.toml

@@ -21,3 +21,5 @@ enableRobotsTXT = true
   twitter = "https://twitter.com/GoHugoIO"
   instagram = ""
   youtube = ""
+  # choose a background color from any on this page: http://tachyons.io/docs/themes/skins/ and preface it with "bg-"
+  background_color_class = "bg-black"