simplify grid-bg-col and grid-bg-col

src/client/styles/scss/_on-edit.scss

@@ -317,11 +317,7 @@ body.on-edit {
 }
 
 // [TODO support dark mode by GW-3037]
-.grid-bg-col {
-  border-right: 5px solid white;
-  border-left: 5px solid white;
-}
-
+.grid-bg-col,
 .grid-editable-col {
   border-right: 5px solid white;
   border-left: 5px solid white;