This is subjective and as old/unsettled an argument as vim vs emacs or spaces vs tabs - although what you're saying is true for you there are many people who argue the opposite. I must have spent at least a day's worth of hours over the years reading long threads with people debating over which is 'correct', 'safer', 'healthier' etc: dark on light or light on dark. People are different. Having a preference is normal but framing it as assistive tech is too much as there are people out there who feel a strain looking at light text on a dark background.
It depends largely on the ambient lighting. Dark themes are hard to read on my bright sunlit train journey and bright themes are often too much for a dimly lit room. I switch between both.
Yes. Healthy eyes depend on natural light or bright near-natural (but low UV) artificial light. Ambient light levels affect pupil dilation and focus, and screen brightness/colors should be matched as ambient light changes. At night, an hour or two before sleep, the blue content of light should be reduced (or lights dimmed) to trigger melatonin production for circadian cycle and sleep.
Summary: use light mode near daytime windows or quality indoor lighting, use dark mode or inverted colors in the dark.