Huh… That’s the first time I ever read that. Usually it’s less painful because there’s less bright lights being shown. White background means your eyes are looking at more lights while the black texts are the one without the lights.
Dark background means less light so overall your eyes are exposed to less light with the white texts on black background which is generally less painful.
Maybe you’re reading the forum outside in bright sunlight? If so, I think it’s understandable since the dark background absorbs so much light that it’s actually harder to read the white texts on dark background under bright sunlight.