I'm not sure I understand what you're talking about. You can edit the text for each slide in the widget settings, just like any of the other options. To change the styling of the text that overlays on top of the images for each slide, you'll need to create new CSS overrides for it; using
Firebug will allow you to "inspect" each element and test any CSS changes you might want to make.
I am taking about changing the size and background color of the text box as well styling the text box contents. So basically it can be achieved only with a CSS override, right?
Currently the widget settings allows to change size and background of the image fader box alone.