it is implemented to use the same css in the editor as in the site so that it can look as similar as possible in the editor as it will look after editing.
In the new release of mojoPortal coming later this week, there will be an extra css class on the body element used inside the editor and this will make it easier to override any css that you need to with this:
body.wyswygeditor { /* your style rules here */ }
Hope it helps,
Joe