Human eyes don't have good night vision, we see best in daylight. Light text on a dark background makes the eye work harder and open wider, since it needs to a

Human eyes don't have good night vision, we see best in daylight. Light text on a dark background makes the eye work harder and open wider, since it needs to absorb more light. When this happens, the light letters can bleed into the dark background and cause halation. Our eyes focus better when the iris is narrow. Additionally, most people are born with some form of astigmatism, a misshaped cornea that blurs vision. For people that have the worst forms of astigmatism, light text on dark backgrounds aggravates the condition. When looking at a light display, the iris closes more, decreasing the effect of the deformed cornea. When using a dark display the iris opens to receive more light and the deformation of the cornea makes halation worse. On the flip side, dark mode helps with floaters, tiny fibers or spots that appear in a person's vision. These are caused by changes to the fluid in the eye which cause shadows to be cast on the retina. Floaters distort vision in light mode. This condition tends to increase with age. Also, people with light sensitivity might be better served by a dark background. Both light and dark mode should be offered for accessibility reasons. I have found that dark mode works best in low light, 100% contrast (both light and dark) can be harder to read with more eye strain, and reading large amounts of text in dark mode is harder. I have several eye conditions, some genetic and some age related, which makes dark mode unusable. I tried using Dark Reader for evening web viewing, but the blurred vision and eye strain was too much. Flux has been a better experience for me.