I'm currently designing a site which uses a dark background, and just out of interest, I disabled CSS and took a look at it. Because it's been marked up semantically, without styles it's still a coherent document, except for the fact that my heading images are white, and because the default background colour is also white, you can't see the headings at all. (The headings are marked up as h1/h2/h3 and then Javascript replaces them with images).
Now I know this is a minuscule edge case of users who don't have CSS, but do have javascript, so this is a very much more theoretical rather than practical question, but should I go back to the old and outdated way of setting background colours:
<body bgcolor="#333333">
..so that you can still see the white images?