The background color for your blog is not located with the other color changes in "Fonts and Colors". Oddly enough, to change the blog's background color requires going into the template--you'd think it would be among the other clickable changes. Some new bloggers may not like the idea of revising the template code, but eventually you will want something bad enough to dare! When the time comes for this change, look for a section resembling this below (from one of my other blogs):
background:oldlace; url("http://www.blogblog.com/harbor/rocks.jpg") no-repeat right bottom;
font:x-small Georgia, Serif;
or, similarly, this (from the code for this blog):
font: x-small Trebuchet MS, Verdana, Arial, Sans-serif;
It's the "body" where you want to make the change or else create the color. Find or alter the color statement--whether you use a number or name doesn't matter. If you have to add the "background: color;", just be sure it's inside the brackets like you see in the two examples above.
See also my post about Fonts And Colors