I'm sure you can change the background, can't find the file at the moment. But you cannot make it semi-transpararent. Perhaps you can make use the transparent green together with a real colour to approach semi-transparent, like this:
* * *
_* * *
* * *
Where * is the transparent colour. This will will only be possible if the background image is a BIF with the first colour as green. Check it in readbif.

Good luck,
Duke John