Sorry, I wasn't clear, I meant the second part of JayDevlin's post, where he suggested putting:
HtmlEditorConfig::get('cms')->setOptions(array(
'content_css'=>'cms/css/editor.css, '.(SSViewer::current_theme() ? THEMES_DIR . "/" . SSViewer::current_theme() : project()) . "/css/editor.css"
));
into your 'mysite/_config.php'.
Hope that helps