The default question mark help icon seems sort of out of place with the default themes. I see the default theme as quite a bit more 'refined' than the help icon.
I kind of like the current icon. O:-)
If it were to change the + and - icons should also change since they are the same basic style as the current ? as well, at least if you want to maintain consistency.
Note all of the icons come from the fugue icon set, there are several ? + - icons available as I recall. We have tried to only use icons from that single set since the author took the time to make sure they worked together (colors/size/style/etc).
:) His work is really good. This question mark one I think is his 'question-octagon-frame' icon. Probably just my eyes or taste I suppose, but the darker blue just does not work for me. He has one called the 'question-button' that I also like. Maybe it is the eight sides I don't find attractive? I might give the unframed 'question-button' a look-see.
:)