Setting the background color in Slideshow

The OP asked how to set the background color in Slideshow as it is not obvious.
Matthew replied:

Locally, I’d superimpose a picture onto a color rectangle:

#lang slideshow
 (define (add-bg p color)
   (refocus (cc-superimpose
             (colorize
              (filled-rectangle (pict-width p)
                                (pict-height p))
              color)
             p)
            p))
 (add-bg (bt "Hello") "green")

To globally set the background, I’d use that in an assembler:

#lang slideshow
 (define (add-bg p color)
   (refocus (cc-superimpose
             (colorize
              (filled-rectangle (pict-width p)
                                (pict-height p))
              color)
             p)
            p))
 (current-slide-assembler
  (let ([orig (current-slide-assembler)])
    (lambda (title sep body)
      (ct-superimpose
       (inset (add-bg (inset full-page margin) "green")
              (- margin))
       (orig title sep body)))))
 (slide #:title "Example" (bt "Hello"))

(via plt)