First, please stop using "portal development" as the subject line for all of your posts. It provides no useful info for someone searching the forum, and it could be glossed over by someone thinking it's the same portal development question that you asked previously.
To change the background color, are you asking about the color scheme or the background color overall?
In either case, you're going to be creating your own theme that has "classic" as it's parent theme. In the docroot/_diffs/css folder, copy the main.css file from docroot/css in there and add an import statement for custom-ext.css.
Create the custom-ext.css and change the body class definition.
For the color scheme, you're going to do a similar thing with the color scheme specific css file(s).
