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.
Summary: use light mode near daytime windows or quality indoor lighting, use dark mode or inverted colors in the dark.