For something as utilitarian as a search engine, being usable by default, and cool after tweaking is probably better than the reverse. I question the logic of having such a setting at all -- it's not just a matter of preference, having the setting is basically admitting that the default design has a (serious) usability issue -- so logically it shouldn't be the default.

It's kind of funny that we went from black on grey (netscape), via black on white (ie4) to grey on white (by design/via css) -- rather than simply back to black on grey or grey on black ("console" colors) -- as there's been some studies indicating that darker background leads to less power usage -- which actually has a usability benefit in longer battery life -- all the while maintaining a lower contrast.