Adium

WebKit

The Apple technology which also powers the Safari web browser. It lets applications make use of a sophisticated HTML and CSS rendering engine for beautiful display of content.

Adium uses WebKit to display messages in a customisable format. It uses plug-in bundles called WebKit Message View Styles, containing HTML, CSS, and JavaScript that tell the WebKit Message View how to render the message. WebKit styles are available in plenty from the Adium Xtras website.

Message view styles and Skype

Skype has a feature that they don't tell anybody about: you can use Adium message view styles as Skype chat styles. So the same message view styles that you enjoy in Adium, you can enjoy in Skype's IM feature as well.

See this blog post for instructions.