The accepted answer in How to change the appearance of a Cell GroupOpener? gives programmatic methods for changing the shape, color, or other appearance of a cell GroupOpener.
I don't see any way to use the Option Inspector to change what dingbat or other symbol is used for the opener. Is there some way to do it by modifying the stylesheet?
GroupOpenerColor, but it has no real effect.GroupOpenerInsideFramedoes have an effect, but it's not what you want I think. There's also this:VirtualGroupDatabut I don't know what it does. – b3m2a1 Sep 26 '17 at 17:38