Empathy has been shipped as default instant messenger on Karmic Koala. One good thing about Empathy is that, by default it supports Adium Message Styles.
First, make sure you have WebKit installed. If not, you can get it through PPA for WebKit Team.
Remember to update and add key for this repo. After that, issue this command in the terminal
sudo apt-get install libwebkit-dev
Now, you are ready to go. You can simply download the style from adiumxtras and manually extract the tarball to ~/.local/share/adium/message-styles/*.AdiumMessageStyle or grab a script for automatically handling adiumxtras protocol right from the browser here.
Make sure to take a look at theme support status section to see whether the theme is fully supported.