I prefer the dark theme, because it is easier on my eyes. I am a professional developer, so I am sitting in front of IDEs for some 7h/day. When I come home, I have to look after several projects of mine, meaning a lot more coding. My eyes tend to become very tired and irritated from the white backgrounds. Also, especially when I am at home and it is dark outside, my room is often not lit brightly, so a white screen would be constantly blinding me.
I don't only have a dark theme for my IDE, but I also use a dark themed KDE and I darken all websites using the Firefox plugin "Stylish".