Whether it is a dock icon or an application, there's a simple way to change the icons on your Mac OS X. And you don't have to use any application for this, as it is kind of built-in feature in Mac.
There are some apps which can help you change the icons, but since you can do it with yourself, using a third-party app is useless. Alike iOS, Apple's Mac OS X is customizable and with Flavours app, you can also apply themes on Mac or you can even create your Flavours theme.
This trick works on Lion, Mountain Lion and it should work on OS X Mavericks too as there are not many changes, but it is not yet confirmed whether it works on Mavericks or not.
How to Change the Icons
First things first, whenever you are changing the icon of any application or any file or any folder, you should backup its icon first so that you can revert back if you want or if something goes wrong.
For taking a backup of your icon, open the folder containing the application or folder and right-click on it and click on "Get Info". You can alternatively select the application or folder and press "Command + I" . Once opened, you will see a big icon of the application under Preview section. Just drag the icon from their to the desktop to make a copy of it.
Now to change the icon, what you have to do is download your favorite icon from web and save it on your Desktop. Now hit "Command + I" after selecting the application and drag the downloaded icon from desktop to the small icon on the top left in the Get Info Window.
This will update the icon but it will show the default icon until you Restart or Relaunch the Finder. Once you restart the finder, it will show you the new icon which you have applied. If you don't like the new icon or if you want to revert back to the default icon, just drag the backup icon from desktop from the very first step to the small icon in the top left in the Get Info window and you will set it to default icon.