It's simply much, much easier for me to work with a dark background. Not fully black, but preferably a bit in the purple direction. You are not top-down reading most of the time, but working on lines, jumping between them. That's much easier for me on a black background, because the text pops out and the background "noise" is much easier on the eyes.
When you're reading a block of text, you interact much less with the background than when you're searching and jumping between lines.
When you're reading a block of text, you interact much less with the background than when you're searching and jumping between lines.