That depends on the source of light. External source of light like a lamp or the sun is better to read black texts on black background because the light source reflects upon the white page giving contrast to the black texts.
But when you’re looking at a screen where the light source is coming from behind like the texts itself is emitting the light. Then the inverse would apply. It’s more comfortable for white texts on black background since the text is giving the contrast.
That’s why Day mode is useful during the day and you’re on mobile. But not so much for indoors where most computer screens are used.