Is a "dark mode" (light text on dark background) really better for the eyes?

As a default, definitely not. It may be good in low light situations or for specific optical disabilities, but it is worse for normal sighted persons and for good reasons grounded in optics.

In an earlier life I worked in optics, designing holographic cameras for bubble chamber physics, so I know a little about focussing systems and depth of field. The iris in the eye expands or contracts to allow or restrict the amount of light entering the eye, attempting to maintain a constant luminosity on the retina so that either not too little, or not too much light falls on the cones and rods in the retina. When the iris is dilated (open) the eye allows as much light as possible into it. You can visualize the light from a point (say a pixel) that reaches the iris as a cone. The lens in the eye focuses this expanding cone back into a contracting one that comes to a point on the retina corresponding to the position of the pixel "out there on the screen".

Focusing is achieved by a muscle around the eye's flexible lens, the cillary muscle, which squeezes the lens into a more spherical shape to cause rays entering the eye to bend more, bringing objects closer to the eye into focus, or by the cillary muscle relaxing, allowing the lens to stretch back to a flatter shape, to bend rays less, bringing objects further away into focus. In old age shortsightedness is caused by the lens losing its elasticity and remaining squeezed, and longsightedness by it losing flexibility and the cillary muscle losing strength so that the lens cannot be squeezed as much. By the time we hit our 40's many of us will suffer one of these two extremes and have to wear glasses either for reading or for driving or, in my case, for both. Now that I’ve had a lens replaced for glaucoma I no longer need glasses for driving (yippee!!).

Getting back to the cone of rays from a pixel that the iris and lens conspire to bring to a point on the retina, if there is a lot of light incident on the eye then the iris is undilated (mostly closed) and so this cone is much slimmer. The result of these differing cones on focus is called depth of field. When the cone is "fat" depth of field is reduced; only pixels in the same plane (actually a spherical surface, not a plane, because the retina is spherical; film cameras have planar light receptors; the eye and camera obscuras etc have spherical light receptors) will be in focus; others out of the plane will produce a diffuse circle on the retina. When the cone is "slim" (because more light is incident on the eye) depth of field is increased because the size of the out of focus diffuse circle is smaller. Consequently, when there is less light falling on the eye, depth of field is reduced; the cones are fatter and as the eye roams the cillary muscle must work to alter the curvature of the lens to keep things in focus.

The implication for mostly dark screens is that, while they appear to have better contrast (they do not; but more on that below), the real effect is that it causes the eye to do more work than a mostly white screen because the amount of light entering the eye is less. So both in the short term and especially over the long term a dark theme, relative to a light theme, will tire your cillary muscle and cause your lens to stiffen or squish sooner.

Why then, if what I'm saying is true, did all those World War Two military aircraft use white letters on a black background? My lotus europa is the same. The disc of each instrument is illuminated by a lamp so one can see it at night, and were the panel painted white then, for the same contrast, much more light would hit the eye than for white letters on a black background and the pilot's (or driver's) night vision would be impaired as the iris would contract. This matches for example the iPhone’s night time dimming feature.

I don't want to fear monger, but I do want to suggest that for most people it is healthier and less wearing on the eyes to use predominantly light themes.