I somewhat disagree, I have fairly strong astigmatism and do best with light font on a black background. It also reduces the blast of Lux that's hitting my eyes when I'm reading in the evening.

BTW, you know our eyes are not-color corrected, the way a decent camera lens would be. That means each color comes into perfect focus at a slightly different distance - and the lens needs to readjust. If you've ever wondered why yellow text on a blue background (or vice versa) makes your eyes tired, it's the constant refocusing.

Personally, I often use green on black, mostly in honor of the many years I spent working on terminals that used P39 (green) phosphor. But I have no issues with white on black on an OLED screen (light emitter is at the front surface, like on a plasma screen) or on an IPS screen (backlit, but different light 'valves')

But try some different colors - or a different monitor...