@infocms
Open the file called style.css in your theme and look for “body” (it’ll be near the top). Modify the background-image & background-color properties to suit your theme.
You could also check out themes that enable you to customize stuff with a few clicks in an admin panel. Just so happens I’ve built a couple
Here’s a link to the freebie: http://buddylite.com/buddylite-basic/
You’re on your way to learning all kinds of cool stuff! Have fun