Hi @marklaane, you are right in that the black background is indeed intentionally so to increase the pupil size and so maximise the chance of a good quality calibration. Accordingly, I doubt that the ability to change the background during calibration is likely to be implemented, but I shall create a feature request on your behalf and pass it along to the developers.
In the meantime, it might be worth turning down the brightness/contrast of the your screen in situations where the application is mostly brightly (or white) coloured.