Make colorless.theme de-color vertical window borders

Fixes #1220.

themes/colorless.theme

@@ -251,6 +251,7 @@ abstracts = {
   # default background for all statusbars. You can also give
   # the default foreground color for statusbar items.
   sb_background = "%8";
+  window_border = "%8";
 
   # default backround for "default" statusbar group
   #sb_default_bg = "%8";