There are mainly
three relevant, scientific facts that has to do with light mode and dark mode.
1) That points of light spread in your retina, while "points of dark" (absence of light) do not.
For this reason, light text on a dark background (dark mode) "bleeds" in your retina, which depending on your vision, becomes blurry and less in-focus, and letterforms begin to run into and merge with each other. This decreases legibility. Squinting reduces the amount of light, and is a common response to increase focus. Squinting is a cause of eyestrain. Turning down the brightness helps but may cause the text to be too dim, by the point where bleed is insignificant.
Dark text on a light background (light mode), on the other hand, has surrounding light bleed into it. This reduces contrast a little bit, but because modern screens already have so much contrast (and black-on-white is usually too high contrast anyways), but legibility remains ideal and there's no eyestrain due to focus problems.
2) That for dark text on a light background (light mode), an overly bright screen relative to the surroundings may cause eyestrain, as you squint to compensate. The maximum brightness on your screen is often too bright unless you're in a super sunny room. To correct this, hold a piece of white office paper next to your screen, and reduce brightness until the screen matches. (If you're watching movies or something, however, you'll probably want maximum brightness.)
3) That lots of light, apparently blue light, before bedtime, can work against you falling asleep.
What's this all add up to?
Well, if you're young with amazing eyesight, you might be able to do whatever you want, but you'll have an easier time if you don't keep things at maximum brightness when working, no matter which mode you're using.
And if it's close to bed then either f.lux or Night Shift are a good idea, with a dim screen if using light mode, or a not-too-bright screen if using dark mode.
But that for normal work during the day, if you have fairly average or worse vision? Light mode absolutely wins the day in terms of eyestrain and legibility, as long as your screen is kept to the brightness of a white piece of paper, and not more.
But if you just like the aesthetics of dark mode and you don't get eyestrain or headaches from any squinting? Then go ahead. There doesn't seem to be any evidence that long-term squinting causes any long-term vision deterioration. But as you get older, you may find yourself needing to switch back to light mode one day.