Solution: You can tweak the CSS-output of the xhtml-export by using inline style. The inline style in this solution is inserted via the HTML option of the Textinsert plugin.1) The inline style must be inserted in the exported page. Take a look at the wiki code of the pages: :livedemos:iframe-yellow and :livedemos:bgnone.
This IFRAME contains the xhtml export of the page: Default Output
This IFRAME contains the xhtml export of the page: BG-yellow
This IFRAME contains the xhtml export of the page: BG-none
To see how to insert the macros, take a look at the wiki code of the pages: :livedemos:iframe-yellow and :livedemos:bgnone
A LiveDemo for: incorrect table display in xhtml export