package = Notebooks[][[2]]

SetOptions[ package, {
  StyleDefinitions ->
       Notebook[{
         Cell[StyleData[StyleDefinitions -> "Package.nb"]],
         Cell[StyleData["Code"], CellMargins -> {{0, 0}, {0, 0}}]}],
  ShowCellLabel -> False,
  ShowGroupOpener -> False}
 ]

 (*notice that we are styling "Code" cell as this is the default one*)