You can change this by right-clicking the Adium icon file you downloaded, and choosing 'Show Package Contents'. This will open the contents of the icon set.

Now open the file named 'Icons.plist', it should open in the Property List Editor app.
Then under the 'List' group go down to the item "typing" and change it's value to "List-Custom.png" (without the quotes).
Then go down to the item "Unviewed content" and change it's value to "List-Typing.png".
Done!
Also change those two items in the 'Tabs' group as well.

I found there are two Xtras with the name 'Traffic Lights' in the title, one is 'TrafficLights' and the other 'Traffic Lights'.

At the time I was making them I didn't realize this, so I changed the actual filename to 'Traffic Lights Status', to avoid my set having the same name, I hope this doesn't prevent me from submitting my Xtra.

I named it after what Steve Jobs called the red, yellow and green buttons on Mac's titlebars.