in the file ckeditor/config.js you can add a CSS class to the body of the editor and then use normal CSS to change the background and font color.
So your CSS may include this:
.editor-body{
background:#000000;
color:#ffffff;
}
and in your config.js file set the editor to use the above class with:
CKEDITOR.editorConfig = function( config )
{
config.bodyClass = 'editor-body';
};