I think tinyMCE has several CSS files it uses. Also Serendipity assigns the Stylesheet of your Blog to the content area as to easier preview it.
As Serendipity uses ".serendipity_body" and other classes for showing it, those might not be defined in your frontend Stylesheet.
Try to change the background color of the body element of your frontend stylesheet temporarily and see if it affects the output. If it does you might need to push around some elements in the frontend stylesheet so that it matches on TinyMCE.
I haven'T looked exactly at how the editor draws its elements from and which element needs to be changed. Maybe you can see if you achieve something, and if you fail, I'll try to help you?
Regards,
Garvin
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