I've always forced myself to take a break from the screen, and for very long (book length) stuff will always either use a hard copy or ebook so perhaps my usage

I've always forced myself to take a break from the screen, and for very long (book length) stuff will always either use a hard copy or ebook so perhaps my usage patterns won't match yours. I use variants of solarized style light themes, though do experiment occasionally as I've never found the definite 'answer' to the best theme for me. I don't think the research would conclude that dark backgrounds are a fad necessarily. The reason I'm attracted to themes with darker backgrounds is that they are generally better for getting a set of text colours with roughly equal levels of legibility - whereas lighter backgrounded themes often have two or three colours which are really hard to see. The flip side is that darker colours + astigmatism seem to lead to reduced breadth of vision.