light text color

Hello everyone,
I have been using drupal for quite a while, but am really new to the wysiwyg plugin.
Almost everything is working really fine, but my only issue is, that on of the themes I use has a dark background and white text. Editing this page with CKEditor, you can't see any text, as both background and text is white. So I am wondering, how I can change the background color (changing text color would be fine, too, but it's not really wysiwyg then anymore, is it?).