Problem with the default theme

Hey guys. I’ve been using WP for a little over a year now and I love it, but the default theme is presenting some problems. I really like the simplicity of the default theme, but after switching to another one temporarily, the default no longer works properly.

Basically this is what happens: the header/footer show up properly, but the background that should be behind the main content of the page doesn’t. It’s just the gray background.

I’ve tried everything I can think of, including completely re-uploading the default theme. Any suggestions would be greatly appreciated.