But I can not find a background option to change the colour with this theme!
See here please . Appearnce > Themes > Theme Options
Under "Color scheme" in the drop down menu you have 6 choices you can make there. None of those include the option of being able to upload your own background. If you want the ability to change backgound colors or to use an image as a background then you will have to purchase a CSS upgrade.
Since there is no official Staff support for the CSS upgrade, and precious little Volunteer support, I recommend that you don’t buy the upgrade unless you have at least moderate understanding of HTML and CSS because you must be prepared to do the work on your own.
If you are considering purchasing the upgrade, you can go to Appearance > Edit CSS. There wordpress has provided a preview function where you can try before you buy.
If you do purchase the upgrade be sure to read this article before starting your editing > http://csswiz.wordpress.com/2009/10/15/if-you-have-the-wp-com-css-upgrade/