change color or create a new Body background color

You just need to override the background color for the body. Look in base-colors.css to find what you need to copy and override, based on what you selected as the background color in the theme configuration settings. If you just selected white, then there is no extra class, you just need to override:

body {
  background-color: #FFF;
}

Follow the instructions in sample.local.css to add your own styles there.

FYI, I am not maintaining this version of the theme any longer and am recommending anybody who is able to instead switch to the newest version, which is based on AdaptiveTheme.