5
The answer that work come from franckblack:
you have to change the file
/class/xoopseditor/mastop_publish/formmpublishtextarea.php
there is already the tag for content_css
content_css : "'.XOOPS_THEME_URL.'/'.$GLOBALS['xoopsConfig']['theme_set'].'/style.css",
change it that way:
content_css : "http://www.yourdomain.com/editor.css",
take the content from style.css for the new editor.css and remove the background from the body (i assume). then you are done.
HTH