I don't know of any e-Ink reader that let's you set the background to black with white letters out of the box. But I suspect the Sony PRS T1 and the Nook Touch (probably not an option in the UK

) could, once they are rooted. Both run on Android and most Android reading apps have a night mode.
But I'm not sure you even need a night mode on an e-ink device. Do you have problems reading normal paper books as well? Because e-ink is a pretty similar experience to paper, just with greyer background. Tose screens aren't backlit like that of your (Nintendo?) DS .