Change css based on the page url
The body element will have a boatload of classes on it.  This does vary by theme, but typically there will be a class like

body.page-the-url-for-this-page


that you can use in your CSS.

Do a view source on various pages and take a look.  Or, use the Inspect Element feature of Firefox's Firebug or Google Chrome, or the Firefox Web Developer extension.

If the class isn't there, you can do something like

function thetheme_preprocess_html (&amp;$variables)
{
  $variables['classes_array'][] = "page-" . str_replace("/", "-", request_uri());
}


You may also want to investigate the Context module, which can aid in section-specific features for a site.