We’ve had mixed results trying to create an off-white ebook page. We don’t recommend doing this, though. It’s almost always wasted time, and page colour is now often determined by the ebook-reading device software for difference contexts (e.g. dim or reversed out for low-light situations).
If background colour is really important to you, the best place to change that is in the CSS file of the epub itself (more about editing CSS later), by adding a background colour to the body section.