Open the download file in ThemePark and press Edit ArtFile. Select titlebarcontrols from the sidebar (do not expand it) and click on the image list on the right side of the window. Select all and copy with ⌘A and ⌘C.
Select File > New and press Edit ArtFile. Select titlebarcontrols from the sidebar (do not expand it) and click on the image list on the right side of the window. Paste with ⌘V and close the window, saving the changes. Apply your new theme and log out.
Mount and open the disk image. You will see two folders there. Open the Toolbar Update folder and select all of the files in it. Drag them to the Resources folder in the disk image. That folder is actually an alias that links to /Applications/Adium.app/Contents/Resources.
If you have installed Adium in a different location than /Applications, right(control)-click on Adium in the Finder and choose Show package contents. Open Contents and then Resources and drag the files from the Toolbar Update folder in the disk image there.
I've decided to use it anyway because pretty much every application I have uses the gear for advanced properties. I'll probably redesign it so it's sharper but not substantially different than how it is now.