NotebookSave[Notebook[{
  Cell[StyleData[StyleDefinitions -> "Package.nb"]],
  Cell[StyleData["Code"], CellFrame -> 3,
    CellMargins -> {{55, 3}, {4, 4}},
    CellFrameColor -> RGBColor[0.1, 0.6, 0.1]]
}], FileNameJoin[{$HomeDirectory, "Desktop", "mypckgstyle.nb"}]];

Export[FileNameJoin[{$HomeDirectory, "Desktop", "dummy.m"}],
  "f[x_]:=x^2;",
  "Text"
];

pckgstyle=CurrentValue[$FrontEndSession, "DefaultPackageStyleDefinitions"];
 CurrentValue[$FrontEndSession, "DefaultPackageStyleDefinitions"] =
   FileNameJoin[{$HomeDirectory, "Desktop", "mypckgstyle.nb"}];

NotebookOpen[FileNameJoin[{$HomeDirectory, "Desktop", "dummy.m"}]];

CurrentValue[$FrontEndSession, "DefaultPackageStyleDefinitions"] = pckgstyle;