I've been using Limechat as my main IRC client for a few months now.

Since I wasn't very pleased with the default theme, I started using the Whisper theme (found on the Google group for Limechat) and I made some tweaks recently. It was quite easy since you can style it through CSS (and a YAML file with a CSS-like syntax). I also had help from Stanley Rost to remove duplicate nicknames.

Try it yourself!