How to change the appearance of a Cell GroupOpener?

Question

I would like to change the Cell GroupOpener appearance.

In the present Mathematica design it is too small and can be missed by a reader unfamiliar with Mma. I know, how to change its color through the OptionInspector.

Now, how can I put there something instead of the check-mark that is there at present? For example, a gray triangle that has been there previously, of something more complex, some graphics of my own design

Edit: To answer the question of Silva below: This is how the Cell GroupOpener looks like at my machine operating on Windows XP, Mma 9.0.1.

Now on my other machine with Windows 7 and same Mma version the GroupOpener looks like a tiny check-mark. It is hardly visible and it is more difficult to operate (i.e. to open and close). For example, visually it is difficult to distinguish the closed from the open one. I am sorry, being presently at the machine with the XP I cannot provide an image for the GroupOpener at Win7.

Now, it is not too bad, as long as it concerns myself, this or that GroupOpener presentation is it. However, I am creating documents to give away, and need that inexperienced people would easily identify the GroupOpener and easily operate with it. So I would like to be able to control its appearance.

Answer 1

According to the documentation, CellGroupData supports Dynamic-object as its status. So with the help of CellGrouping -> Manual, we can design our own "CellGroup opener". It can be a Button or EventHandler placed in CellDingbat, CellFrameLabels, etc. Here is a simple demostration (tested in Mathematica 9.0.1 and 10.0 pre-release on Windows 8.1):

openerIconOpen = 
  GraphicsBox[{RGBColor[0.26, 0.59, 0.26], 
    PolygonBox[{{0, -0.25`}, {-0.3`, 0.25`}, {0.3`, 0.25`}, {0, -0.25`}}]}, 
  ImageSize -> 15, PlotRange -> All];

openerIconClosed = 
  GraphicsBox[{RGBColor[0.8, 0.16, 0.17], 
    PolygonBox[{{0.5, 0}, {0, -0.3}, {0, 0.3}, {0.5, 0}}]}, 
   ImageSize -> 15, PlotRange -> All];

cellopenflag$1 = True;
cellopenflag$2 = False;

Clear[openerBoxFunc]
openerBoxFunc[flag_] := TagBox[
                               DynamicBox[If[flag, $CellContext`openerIconOpen, $CellContext`openerIconClosed]],
                               EventHandlerTag[{"MouseClicked" :> (flag = ! flag)}]
                              ]

cellBodys = {
             "This is a Text Cell.",
             BoxData[RowBox[{"Another Text Cell with GridBox: ", GridBox[{{"a", "b", "c"}}, GridBoxDividers -> {"Rows" -> {{True}}, "Columns" -> {{True}}}]}]],
             BoxData[RowBox[{"1", "-", "2"}]],
             BoxData[RowBox[{"-", "1"}]],
             BoxData[DynamicBox[GridBox[{{"cellopenflag$1", "cellopenflag$2"}, {cellopenflag$1, cellopenflag$2}}, GridBoxDividers -> {"Rows" -> {{True}}, "Columns" -> {{True}}}]]]
            };

Notebook[{
   (* Start of group 1: *)
   Cell[CellGroupData[{
      (* Cell 1: *)
      Cell[cellBodys[[1]], "Text",
           CellDingbat -> openerBoxFunc[$CellContext`cellopenflag$1]
          ],
      (* Cell 2: *)
      Cell[cellBodys[[2]], "Text"],
      (* Start of group 2: *)
      Cell[CellGroupData[{
         (* Cell 3: *)
         Cell[cellBodys[[3]], "Input",
              CellDingbat -> openerBoxFunc[$CellContext`cellopenflag$2]
             ],
         (* Cell 4: *)
         Cell[cellBodys[[4]], "Output"]
         },
         Dynamic[$CellContext`cellopenflag$2]
        ]](* <- End of group 2. *)
      },
      Dynamic[$CellContext`cellopenflag$1]
     ]](* <- End of group 1. *),
   (* Cell 5: *)
   Cell[cellBodys[[5]], "Text"]
   }(* <- End of Notebook body. *),
  CellGrouping -> Manual,
  ShowGroupOpener -> False
  ] // NotebookPut

Answer 2

in the absence of a suitable answer to change the appearance of the cell opener and if the cell does not require user edits, you can set WholeCellGroupOpener to True (Format menu - Option Inspector under Cell Options (ensure you have the cell selected)) or by using the Classroom Assistant palette in the Content drop down menu in the Writing and Formatting section and avoid using the cell opener all together.