Exporting in EPUB results in no margin with similar results described by http://epubsecrets.com/break-rendering-css-body-width-100-percent.php
Is there an EPUB CSS quick fix for body margins?
Impossible to determine without knowing what theme you are using.
We are using the McLuhan theme. However, margins in EPUB may be related to the reader app. Currently VitalSource Bookshelf 8 Mac does not have margins in the default EPUB 3 export.