As you may know, you can change the pop-over box size and backgorund color. As well as chanfge the text properties inside the box. The box line (stroke) can also be changed. But unlike the point of the of the bubble shape, you can't change the pointer that connects it to the it's image.
Having been introduced in October with IBA 2.0, the pop-over widget is still in it's first version, I'm afraid. It's very limited. I also hope that we'll have the ability to customize it in future versions of IBA.