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.