I have a wrapper function that I use to init the RTE. Try doing something along these lines, which is similar to what @MiniRagnarok posted.
I just ran a test and it changes the CSS in both the editor's format and the inside the editor's contents. Be aware that you would need to reference the stylesheet on the page that the content is being display on or you won't see the updated styles.
function rteInit(inputId, height) {
var editor = CKEDITOR.replace(inputId,
{
contentsCss: '/admin/css/ckeditor.css',
height: height,
toolbar: [ <!-- Toolbar options --> ]
});
}