[Update – Post obsolete: Appearance > Background now includes a “Restore original bg image”, as it should.]
In Dark Wood, you can insert a background image of your own, by uploading it under Appearance > Background (but see here).
Under the same section you can also remove the image you have uploaded, of course. But doing so will result in no background image at all: the option to revert to the default look of the theme was under Appearance > Custom Header, but that section disappeared a few days after the theme was launched (because its header image function was faulty).
So, to restore the original look, you’ll have to re-upload the original background image after you copy it to your computer; here it is: