I have the mini and I use it for night readind with epubs and the night theme. It's a feature that put the text in white and the back in black (It comes from Android to be honest). If you set the light parameter a little down too, it's very confortable to read and I don't feel any eyes strain, even after a long read. It takes a short period to get acustomate to, but it's very nice.
It works only with epub, not pdf, so I keep the pdf for day reading or shorter reading periods, but, with a low light set in parameter (quite low, it's closer to e-ink this way), the strain is not so strong. you just have to put it back up when you want to use the mini as a tablet again