Definitely dark, I find my eyes feel more strained looking at themes with a light background.

My go-to for many years now has been the Wombat scheme (originally for vim here https://dengmao.wordpress.com/2007/01/22/vim-color-scheme-wo... but you can find ports to other editors such as Visual Studio, Eclipse, XCode, etc if you hunt around a bit).

Dark, for sure. I actually really like Ryan's railscasts theme [1] and I'm now using it for everything.

[1]: https://github.com/ryanb/textmate-themes

Light unless I'm in a completely dark room and feel like my eyes are being radiated.

I always feel that way when I try a light theme, but I wonder if it is a matter of habit. For example, when I work with eclipse for a long time, I tend to forget that I'm using a light theme.

Speaking of monokai, I recently switched to molokai and am enjoying it.

vim = https://github.com/tomasr/molokai

Xresources (e.g. xterm) = http://delicious.com/redirect?url=https%3A//gist.github.com/...

iTerm2 = https://github.com/larssmit/iterm2-getafe

I solely use dark themes for editors and terminals.

I switch between light and dark themes. I am often working on more than 1 thing, and I think it helps me mentally separate them by having different background colors for different tasks.

White(or verly nearly so) background. With plenty of light, preferably near large windows. I can't understand why anyone would ever want to sit in the dark staring into a lightsource.

I use a dark theme for almost everything but I use a light, high-contrast theme when I'm working in direct sunlight or any other very bright conditions.

Dark for code, light for prose.

Currently wombat and solarized-light, but I seem to like to change these every couple of months or so.

Light, though only with off-white backgrounds.