To use Shoreline you must have OS X 10.5 Leopard. The app hasn't been tested with other versions of the MacOS and will probably not perform as expected on older versions.
It should also be noted that in order to work it's magic, Shoreline has to modify certain critical system files (Namely the Dock Application). The program takes every precaution when making these changes and will allow you choose to keep the changes or not after you hit "Preview / Apply".
That being said, there is a possibility (however remote) that the program might damage your Dock Application. It is always a good idea to make an extra backup of system files before changing them. You can make a backup of the Dock Application with the following steps:
It will put an icon on your desktop called "Dock.zip" this is a complete backup of your currently working dock. If there are any problems, unzip this file and copy it back into the "CoreServices" folder, restart your mac and you should be back up and running.
Now that you have a good solid backup you can feel even safer playing around with different Dock Themes.