I can give you a short tutorial that is explaining, how to change the textbox.
First of all, you need to research the tileset of it.
When I'm finished with researching, I get the offset 41F1C8.
Open your tilemolester and jump to the offset.
Now you can edit the set as you wish!
After that, you want to change the palette too, for sure
Again after some researching I get 471DEC for the palette.
Enter the offset with APE or change with the hexeditor. :p