//HtmlHelper Grid
@(Html.Kendo().Grid<OrderViewModel>()
     .Name("grid")
      ...
     .Editable(editable => editable.Mode(GridEditMode.PopUp).Window(wnd => wnd.HtmlAttributes(new { @class= "my-custom-window" })))
)

//Kendo UI for jQuery Grid
  $("#grid").kendoGrid({
    ...
    editable: {
      mode: "popup",
      window: {
        attributes: {
          "class": "my-custom-window"
        }
      }
    }
  });