In older versions of mojoPortal you solve that as described in step 7 on this document.
In newer versions of mojoPortal there is a new class on editor body (as of version 2.3.6.4) making it easier when you need to override some of the styles in the editor, you can do it with:
body.wysiwygeditor
{
color:black;
background-color: white;
background-image: none;
min-width: 100px;
}