Zoho Writer's latest update enhanced the setting up of margins. Clicking the 'Document Properties' button, one can set the top/bottom & left/right margins.

For user convenience, the left/right margins have now been made draggable. Please note that the margins are calculated as percentage of screen area. Try the new feature out.

ps: From the feedback we have received so far, setting the margin properties globally is one that's been most requested. We will try having this soon