Hi Mauno,
I do not see any influence of the theme being used on the actual font size etc. of the text typed when in TinyMCE editor. The current theme's CSS is only applied once the text has been saved and is displayed in a "page", forum discussion, etc. in the current moodle course outside of the editor.
Do you mean in your post that you think this not the correct way things ought to work in 2.0?
But ... the "output" of the text typed in the TinyMCE editor will be displayed in various resources and plugins, in places which have their own CSS rules.
So, in order to get a perfect WYSIWYG match between (a) what you type, (b)what you see in TinyMCE and (c) what you will finally see in the resource or plugin, TinyMCE should be provided with a mechanism to know the nature of the resource that was current when it was invoked.
Do you agree? and is that clear? I have no idea if it's feasible, however.
One more thing: TinyMCE toolbar features a Styles dropdown list which is empty. What's that for?
Joseph