The easiest way to do this is to use "plone.app.theming". But if you need to integrate Plone with other back-end servers, legacy systems, or any webservice, you can use Diazo to all combine it in a unified look & feel.
- plone.app.theming manual
- Diazo theme engine guide
- Quick Test Recipe
- Create a Plone theme product with Diazo
General information on the stylesheets and other resources in Plone
Information on 'old-style' theming for Plone 3, but still valid in Plone 4. So upgrading from Plone3 to Plone4 is possible without switching to the new style of theming.
Do note that for all new theme development, Diazo is strongly recommended.