Honest question: how well-lit is the area you use your computer in, and have you tried turning down your monitor's brightness? I'm as baffled by the popularity of dark themes as you are with light themes, but I'm sitting in a fairly bright room with natural light and a monitor at 35% brightness. In these conditions dark-on-light and light-on-dark text are equally pleasant to read, and I opt for light themes when given the choice simply for consistency. If you're using dark themes because you browse the web in the dark, it seems to me like changing your conditions is a better solution for your eye health.