The Structure

This image depicts the basic structure of a Plone site from the viewpoint of a logged in user. Plone does a great job of separating structure, presentation and content. :

Note:

You should be able to do most (if not all) of your modifications through the CSS without having to modify any templates.

Note:

Plone is designed so that any CSS customization made in ploneCustom.css (located in site/portal_skins/plone_styles) will take precedence. So when you modify any CSS elements, move them into ploneCustom.css and leave the originals alone. ,-)

Note:

To determine which CSS is being used, look at the template code and search for class= and id=. The name in quotes after them is the CSS that is pulled from the CSS file and used. ,-)