So let’s say you have a shiny new set of icons that you love and you want to keep using. You’ve managed to change every icon in your dock, even the Finder icon. Now you want to know how to change the iCal dock icon. When you do it the normal way it just changes back to the default icon once you open it. When it’s closed, it shows the wrong date. How do you fix this, you wonder? Well, here’s how!
I’m going to use the Litho set of icons I got from The Icon Factory and DeviantArt.

Step 1: Acquiring the icons you need
You will need to get icons to be like the ones above, i.e. an empty one named App-empty.icns and one to be used as the application icon named App.icns. Chances are, it’s pretty difficult to download them like this, so you might have to make them from icons you find. If you make the icons from scratch, make sure you leave enough space for the date.
To make them into ICNS, first find appropriate images to use, as described above. So you found the images you want to use, they’re probably in folder icon format (an icon format that is not going to work in this instance) or PNG. There are a variety of ways to do this, but I used the program img2icns to make these into *.icns files.
You may need to change them from Folder icon format to PNG then to ICNS, depending on what format they’re in.
Step 2: Installing the acquired icons
So, do you have an icon named App-empty.icns and another named App.icns? If yes, please continue. If not, please go back to step 1 and try again. You must have these icons to continue.
Put these icons in a safe place on your harddrive. Once you’ve put them where you want them, control click on them and click “Get info,” or simply press Command+I while selecting them. You want to click the checkbox next to “Locked” so that when you drag and drop them, they get copied instead of moved.
Go into applications and control click/right click on iCal and click “Show package contents.” The package will open. Go into Contents then Resources.
You need to do two things to change the icon properly:
a) Drag the newly created (and locked) icons into the Resources folder. You’ll need to be an administrator to authenticate and overwrite existing icons. This will take care of the application icon in the Applications folder, as well as the icon when iCal is opened in the dock.
b) If you’re running Snow Leopard, in the Resources folder, find the folder iCalDockTilePlugin.docktileplugin. Go into Contents then Resources of this folder and drag and drop App-empty.icns in there. This will take care of the Dock icon when iCal is closed. If you’re not running Snow Leopard, I’m not sure what you’ll need to do. If you can figure it out, please post it in a comment here.
There you have it! This should work. If you want to use the litho icons from Icon Factory, I’ve got the ICNS files I’ve made available for download here.
