TinyMCE sets up the content to be edited in an iframe and loads a stylesheet called @@tinymce-getstyle, which is actually a browser view that includes the stylesheets that are registered in portal_css.
I've had the most success with Diazo when I register my stylesheets in the Plone stylesheet registry (portal_css), and then add Diazo rules to copy them into the head of my theme.
Disadvantage:
- You can no longer edit the theme on its own apart from running via Plone.
Advantages:
- The stylesheets are automatically merged and compressed.
- TinyMCE can load the styles.