Don’t want to wait for Twitter to update Twitter for Mac with the new icon? I wouldn’t want to either, it’ll probably be a few months. In the interim, you can download the updated Twitter for Mac icon here, courtesy of my genius Photoshop skills.
To add it to Twitter, simply navigate to the application, right click on it, and select “Show Package Contents.” From there, navigate into the “Contents” folder, and from there the “Resources” folder. Inside, you should find a variety of Twitter for Mac icons. Rename the default icon, which should be called “Icon.icns” to “Iconbackup.icns” for safe keeping. Then you should be able to drop the new icon in. OS X will ask for your password, and you’ll be good to go. In order to see the updated changes, you’ll have to quit Twitter and relaunch it. In some cases, you may also have to remove the app from your dock and re-add it, and in some fringe cases, restart Finder.