Hi,
The thing to understand is that the editor is using an iframe that has a simple html wrapper and it has <body class="wysiwygeditor">
So the problem is if your style rules depend on some nested divs with css classes like <div class='modulecontent' etc, there are no such divs in the iframe so those styles are not applied.
You can add css specifically to style things as you like in the editor but you will need to use selectors that will work inside the iframe ie:
body.wysiwygeditor h3 {}
body.wysiwygeditor ul {}
etc
Hope that helps,
Joe