The slide generation script creates a series of linked HTML pages. The page markup is generated from templates and the styling is applied using a CSS stylesheet and related images. The templates, the stylesheet and the images can be edited to create the look you're after.
Each of the themes listed here provides a set of templates, a stylesheet and images which you can drop in to your presentation to change the look. Simply download a tarball and unpack the files in your existing presentation directory. It is recommended that you commit your presentation files into your source control system (subversion etc) before installing a new theme - that way you can roll back easily if required.
Inspired by a theme from the LaTeX Beamer package. It's got a seriously boring corporate feel, but with orange highlights.