One of the main reasons to face rendering of CSS is when you need to change the background colour of the portal status message, as its colour can't be changed from themes manager style definitions. To change the styles of the portal status message do this:
- Go to the ZMI
- Go to the tab Find of portal_skins.
- Search for the identifier stylesheet_properties
- Click the one file listed that has a red asterisk (this means that's the one that is being used by Zope).
- Click the button "Customize".
- Search for the property messageBackground and change its default value (#ffca31) for the new one that suits your design.
If you want to know a little more about how CPS uses these properties, do again a search for the file default.css and you will see that the code is in fact DTML which uses stylesheet_properties as variables inside CSS definitions.
And that's all for now, I hope it helps ;-)