Plone theming obviously has some community tools that have been created to make the theming process easier, but it's also possible to incorporate some advanced technologies. The following are a couple of non-Plone-specific products that can be used to alter the look and feel of your site.
sIFR (Scalable Inman Flash Replacement) is a technology that allows you to replace text elements on screen with Flash equivalents. It is explained in depth at http://wiki.novemberborn.net/sifr3/, and can be seen in action at http://www.dogwoodinitiative.org (see the portlet header elements).
The idea is to use fonts that are embedded within a Flash file and render those on a web page instead of standard text. The process is actually quite simple, but has some delicate syntax that can be a bit painful to get right. It's best to follow the source code found in the official demo (http://dev.novemberborn.net/sifr3/beta2/demo/), and to refer to the discussion forum if...