I have wondered for a long time about the hype around dark themes for this reason. No eye is perfect in terms of optical quality. Specifically, the lens can be misshaped and the vitreous can have impurities that lead to refraction of light.
If we look at bright light sources the pupil is almost closed, which means all light entering propagates close to the optical axis of the eye, where lens defects are not playing a huge role.
The less intense the light source gets, the more the pupil opens and more aperture is used. That means lens defects start to become more apparent and the probability the light crosses a part with inhomogeneities in the vitreous part becomes larger.
With dark themes our pupils open more, using a bigger aperture of our eyes, which allows small aberrations like astigmatism to get pronounced. These optical aberrations can lead to double vision (cover up one eye and look at the white letters on a dark background from far), for which our brain compensates when both eyes are used.
Of course, too much light isn't the answer either.
I'm not sure that this is the sole reason why light-on-dark is bad with astigmatism: because, presumably, the article about keynote slides was including presentations given in lit rooms—and eyes famously don't adjust too fast, so I don't think the pupils would react much to dark slides in a lit room. But that's only my conjectures.
