Colloquy (http://colloquy.info) is a very nice IRC client for Mac OS X. It does, however, need some serious help in the area of useful chat styles.
This theme is a modification of Succinct for Colloquy which is based on Campfire's design.
Succincti is made of several small changes from Succinct. The biggest is: colors from outgoing and highlighted messages have been inverted. (the "i" at the end of "Succincti" is for "inverted").
Copy the Succincti.colloquyStyle file to this folder in your home folder: Library -> Application Support -> Colloquy -> Styles
Start or re-start Colloquy. (Typing "/reload style" in a Colloquy chat window may work, but is buggy.)
Select the “Succincti” option from the Style drop-down.
Choose the “Appearance Preferences…” to set Succincti as your default Colloquy chat style or to customize the colors via its “Customize Style…” button.
This theme is based on the work of John Albin: https://github.com/JohnAlbin/succinct-for-colloquy.
Modifications by Thierry Régagnon.