The built-in Acrobat toolbar icons have hover effects like this:
where the highlight icon turns yellow when you hover over it with the mouse. Is there any way to accomplish this with custom add-on buttons? I didn't see anything in the documentation for app::addToolButton()...
Unless someone knows of an undocumented parameter that allows you to specify an on-hover icon, I don't think it's possible.
AFAIK this does not exist