If I am understanding this correctly, the information Koasati said is correct, but that is the second half of the action you need to take. The first part would be to download and unzip the theme you want to use (obviously). Then you need to move the theme folder to your ObjectDock background library.
It should be something like:
C:\Program Files\Stardock\ObjectDock\Backgrounds\Non-Tabbed
After you have done this restart OD then right click in an open space on the dock (somewhere around a separator always works for me). Then go to 'Dock Settings', click the Appearance category header, then select the theme that you just added to your OD library folder under the 'Background' dropbox.
Does this make since?