I am sure we have all tried reading on a smartphone or tablet and the black text on bright white background can be straining on the eyes. (goodereader.com)
Ok definitely checking out the tutorial on how to get bright white backgrounds because I literally have no idea and have been trying to figure it out! (meetat-thebarre.com)
There is also a number of options such as Butter, which makes the background a milky white that is lower on contrast than the default bright white background. (goodereader.com)