Margins in a LaTex document

Yesterday I wanted to modify the margins of a LaTex document: the first thing I tried was simply to modify the following variables:

\oddsidemargin 0.0in
\evensidemargin 0.0in
\textwidth 6.5in

However, this modifies only the width of the document's body but not the width of the header and the footer (I am using the package fancyhdr).

It is possible to fix this problem using the package geometry, like this:

\usepackage[body={6.0in, 9.2in},left=1.25in,right=1.25in]{geometry}

The document attached to this post is a LaTex "template" I use for my the exercise series at school.

Centering elements using CSS
I found this useful page: Centering things.