Actually, I wish I could have found something like that for Button elements specifically, but it seems that I can only set the Transition Type for the whole Menu.
The only Appear Type I can see is the one in the Menu properties which is not related…
This may not be the solution (also, you may have already tried it) but: have you tried setting the Filter Mode of the PNG to "Point (no filter)" and the Format to "Truecolor"?
(Image)