[plt-scheme] changing background in slideshow

Dear PLTers,

I'm looking for a better tool to write my slides, and I am getting
interested in slideshow. However, there is a feature that I need:

(how) can I get slideshow to display a different background depending
on the slide I am showing?

a different color would be a start, ideally, a different vector
graphic specified in a lispy language would be even better.

(I prepared a presentation earlier this year using rst2s5, but its UI
doesn't allow for this kind of trick that I know of, and Alan Bawden
suggested that this particular presentation would be much better
understood if I used color to contrast slides with the "bad" point of
view from slides with the "good" point of view, and slides with an
even "better" point of view.)

[ François-René ÐVB Rideau | Reflection&Cybernethics | http://fare.tunes.org ]
We [the French] will accept any master who will let us enjoy the good life,
with good food, romance and long vacations; and we'll use that good life to
corrupt whoever will rule us into embracing our way of life. Yet we'll
abandon him for a stronger master the moment that his weakness is apparent.