Beautify whitespace highlighting

The current whitespace faces make whitespace-mode too distracting to
read with too many background colors. This commit removes the annoying
colors for whitespace-space, whitespace-tab and whitespace-indentation
because:

- whitespace-tab and whitespace-space already use characters for
  visualizing their appearances. whitespace-tab uses ">>" while
  whitespace-space uses ".". Adding background colors is redundant and
  annoying. Editors like Notepad++ or Vim doesn't use background colors
  to depict such whitespace characters.

- According to the documentation of whitespace-indentation:

"Symbol face used to visualize 8 or more SPACEs at beginning of line.
Used when `whitespace-style' includes the value `indentation'."

It is used for highlighting first 8 characters or some customized
value. We must also disable its background colors otherwise our buffers
are still filled with annoying colors all over the places, since often
indentation is around 8 spaces or less. Anyway, with whitespace-space
and whitesspace-tab, such highlighting like whitespace-indentation is
redundant and not needed.

spacemacs/packages.el

@@ -2716,7 +2716,15 @@ It is a string holding:
                             :documentation "Globally display the whitespaces."
                             :evil-leader "t C-w"))
     :config
-    (spacemacs|diminish whitespace-mode " ⓦ" " w")))
+    (progn
+      (set-face-attribute 'whitespace-space nil
+                          :background nil
+                          :foreground (face-attribute 'font-lock-warning-face :foreground))
+      (set-face-attribute 'whitespace-tab nil
+                          :background nil)
+      (set-face-attribute 'whitespace-indentation nil
+                          :background nil)
+      (spacemacs|diminish whitespace-mode " ⓦ" " w"))))
 
 (defun spacemacs/init-window-numbering ()
   (use-package window-numbering