Colloquy theme highlighting mIRC and available for free to customize the user interface.
The installation process for mIRCish is pretty simple. All you have to do is unzip the file and move it to the ~/Library/Application Support/Colloquy/Styles/ directory. If that directory doesn't exist, you'll need to create it. Once you've done that, open up Preferences and go to the Appearance tab. From there, select the new style from the Default Style dropdown.
If you want to switch back to the Default style in a chat room, simply use the Style dropdown menu in the toolbar. And if you want to use mIRCish in specific rooms, you can select it from the Style dropdown menu on an as-needed basis.
Overall, mIRCish is a great option for anyone looking to change up their chat style. It's easy to install and adds a nice touch to your chatting experience.