LimeChat is a decent IRC client for OS X. You can get it at http://limechat.net/mac/
Solarized is a color scheme that can be applied to just about any application. You can read more about it at http://ethanschoonover.com/solarized
Installing these color schemes is easy. Just clone the repository and copy the files to Library/Application Support/LimeChat/Themes in your home directory.