Use the Body Widget (dashBody: an adornment, so it cannot open as a Text Window) to specify the default optional non-white background color, open the large background adornment (the white rectangle that contains everything) and set its $Color to anything other than #fff. Doing this does not activate the background color, it only sets it.

Doing this sets the default background color. (You can override this by setting $bpBkgColor of the prototype, category, or post you want to affect to a different value.)

To utilize the color you set here, add BKG-COLOR to the $blogExpOpt of the prototype, category, or post you want you want affected.

Activating background color will make the following changes:

• the main column of your blog will become lightly bordered, and
• the main column will receive a 15px padding.

For information on background image and texture, see here.