The website isn't well maintained anymore since we've kinda passed the days that people used multi-service clients. Here are some screenshots:

https://www.adium.im/screenshots/images/overview.jpg

The area inside the chat window is HTML rendered via WebKit and you can change the theme easily. The Contacts list is also HTML in the middle so that's all theme-able too.

https://images.six.betanews.com/screenshots/1110403338-1.jpg

Here's a Contacts list that's more compact and a chat window without the top toolbar.

https://adium.im/screenshots/images/overvieworange.jpg

Here's a more radical theming where they've gotten rid of the window around the contact list so it's floating.

If you go through the Adium Xtras, you can see all sorts of chat (https://www.adiumxtras.com/index.php?a=search&cat_id=5&sort=...), contact list (https://www.adiumxtras.com/index.php?a=cats&cat_id=4&sort=do...), and other ways of styling Adium.