The stylesheet (/includes/templates/your_template/css/stylesheet.css) is the place for that work.

The body {} declaration covers the entire site, and other selectors are used for individual parts of the page.

Look at www.w3schools.com if you need more info on how to use CSS, and get Firefox and its Web Developer extension to help with finding the element tags that control items and editing the CSS. The Information > Display Element Information button will become your best friend.