style["Some text","Text"]//shadow[#,#]&

nb = (*whatever notebook object e.g. EvaluationNotebook[]*)

Module[{cell, cellObj}
, cellObj = #
; cell = NotebookRead[cellObj]
; If[
    StringQ @ First @ cell
  , cell = ReplacePart[cell
    , 1 -> Replace[cell[[1]], s_String :> BoxData@ToBoxes@Overlay[
        { s, Pane[Style[s, GrayLevel@.8], FrameMargins -> {{3, 0}, {0, 0}}
               , ImageMargins -> 0
             ]
        }
      , {2, 1}
      , BaseStyle -> {ShowStringCharacters -> False}
      ]]
    ]
  ; NotebookWrite[cellObj, cell]
  ]
] & /@ DeleteCases[
  Cells[nb], Alternatives @@ Cells[nb, CellStyle -> {"Input", "Output"}]
]