@Ian_Fry It depends on the medium. Backlit devices, often times it's light on dark. Most other application, it's dark on light.

That being said, the reason most web material is dark on light is because it's what people were used to reading. Now we know better, but the decision appears to have already been made. Personally, my older eyes hate going from light-on-dark to the reverse. It's painful. Going from dark-on-light to the reverse is fine.