A child theme is almost never necessary unless you need to do advanced changes in the php templates.
Most customization can be done through the theme options, its many different Custom CSS locations, and even actions and filters in the “plus” version.
ALL these settings are saved in the database and are preserved in any update. For security we advise that you create a backup in the database anytime you do changes, by clicking Save / Restore > Save, and that you also download a local backup using the download ALL settings below that.
Regarding having different background color on different pages, you can read the thread for Weaver II below as it also applies.
http://forum.weavertheme.com/discussion/2648/changing-the-background-image-for-a-specific-page
If you need more help, give us a link to the page where you want a different background, and be specific about which background you wish to change