Publisher is not a code editor.

Depending on what you are trying to achieve, you can add a background color
or texture. Format > background, and potentially More backgrounds...  Don't
try to use a master page with a Publisher webpage...save it for print
documents, as it tends to mess things up in a web page.

You might also get some ideas from this article: Understanding background
padding in a Publisher web (aka white space) :
http://msmvps.com/blogs/dbartosik/archive/2006/01/07/80563.aspx
It is about a workaround for not being able to center a Publisher webpage,
but it might give you some ideas.

If you want to be a bit more specific about what you are tying to achieve,
perhaps we can give more specific advice.

DavidF