Adium is an IM client for mac. It has the ability to add custom message style so you can have your chat window look exactly how you want it to. These are built using CSS and HTML so anyone with any webdesign skills can make one pretty easily.

I've made a simple and clean one. It can be downloaded from adiumextras.

It's entitled 'For James' as I initially made it for myself but my housemate James saw it and wanted to use it himself. It's been designed for you to choose your own background colour which you can do when you set the style. Enjoy!