IMHO, backgrounds don't have to be simple; they just shouldn't be loud. You can have a busy/complex background with subdued colors and tones. The key is to make sure the background doesn't overwhelm or take over the journal.

I think the only time the background needs to be simple is if you have a busy, loud, or very large header image, or if the content boxes (the parts of the journal that contain the entries, calender, tags list, etc.) have no background color.