The textbox.png file isn't used for dialogue in this pack. The dialogue background is setup to use the frame.png file instead. So it's that one you want to change inside the GUI folder. Hope that helps!
Edit: The dialogue background has been set to the "textbox.png" file since this comment, using a tiled Frame() displayable so it can be resized without stretching. Adding this now so anyone else reading this will be informed.