Discard background color on outline-1

.emacs

@@ -1029,7 +1029,7 @@
                 (prettify-symbols-mode)))
 
   :custom-face
-  (outline-1 ((t (:height 1.9))))
+  (outline-1 ((t (:height 1.9 :background nil))))
   (outline-2 ((t (:height 1.6))))
   (outline-3 ((t (:height 1.4))))
   (outline-4 ((t (:height 1.2))))