Later research showed that on average people read better on lighter backgrounds because mild uncorrected astigmatism is very common, and more overall illuminance reaching the eyes causes the pupils to contract, which helps mitigate the astigmatism.
I'm not sure if there has been a study done on people confirmed not to have astigmatism.
It wouldn’t surprise me if there is a evolutionary reason for us being better at resolving small dark things better orso ;)