There are two variations of it– WordPress, and WordPress Busy. The only difference is that the latter hides events (join, part, nick, etc), which I find very useful for very busy rooms. Hence the name.
To install download the zip file containing the styles and image, unzip and drag the variants folder into ~/Library/Application Support/Colloquy/Styles/
From there you should be able to activate the style in Colloquy from Style > Standard > WordPress / WordPress Busy