Philippe--it's in the menu (and in the ~/Library/Preferences/LimeWire/Themes folder), but choosing it gives the "out-of-date" message and the link to update. I used your link, but for OSX I had to change the extension to zip, unzip it, and add the folder to /Themes. Looks like OSX needed the lwtp file and the unzipped folder to work. The lwtp file alone wasn't enough.

swimkid--the Pro theme uses a faint green and the LW plug rather than lime (I don't really notice much else different). You should donate to Pro anyway
Mail me I'll send you the lwtp (if no one objects) or a magnet of a screenshot. Just curious--why wouldn't you get a Pro? You are interested in p2p, and LW development is the most active and interesting as you've seen.