Currently the default theme has a white background, and the dark theme has a black background.

IMHO, they aren't two stellar choices: they both have an extremely high contrast which hurts the eyes when you try to read, it's not relaxing at all.

The best colours I've found so fare are either black text on light gray background, or light gray text on dark gray background. The exact shades may vary.

I understand the urge to have a white background anyway "just because google does so", and I won't complain, but could you please at least adjust the one for the dark theme?

I believe everyone would feel better then, even if they don't realise it at first.

Google has nothing to do with our decision to use white background. It's a pretty standard design. As for tweaking the dark theme. I think that we prefer to have the text be high contrast since that makes it readable for everyone. If you'd like to make it more gray, then I think the best approach would be to use some kind of user stylesheet. There are no plans to make it customizable right now, but it could be a cool thing to add later.