Resource Registries

This step let's you register CSS, KSS and JavaScript resources into the the corresponding registries, defining them in cssregistry.xml, kssregistry.xml and jsregistry.xml, respectively.

Let's see an example with the CSS registry. This step lets you register stylesheets in the portal_css tool. This tool is a site-wide registry providing interesting features like conditional inclusion, caching, merging or compression. Check the Resource Registries section in the Theming Manual for more info about them.
An example registration in the cssregistry.xml file would be:

<?xml version="1.0"?>
<object name="portal_css" meta_type="Stylesheets Registry">
 <stylesheet title="" cacheable="True" compression="safe" cookable="True"
     enabled="1" id="pro_members.css" media="screen"
     expression="not: portal/portal_membership/isAnonymousUser"
     rel="stylesheet" rendering="import" 
     insert-before="ploneCustom.css"
     />
</object>

This code will slide in the pro_members.css stylesheet just before the ploneCustom.css one (so overriding its values if there are any collisions) when the user is not anonymous.
Check the Resource Registries section of the Theming Manual for more info about the different options (XML attributes) available.