After find your desire background, download it.

Uncompress the zip file in a new folder so you will obtain the files to continue. For example in this case we have the background and the icons.

Pressing the "Control" key... make click at the files "background.ini" and "hypergrass.png"....

Click right and choose "Cut".

Go to C:/Program Files (x86)/Stardock/ObjectDock/Backgrounds/Tiles

Create a new folder called "Hypergrass".

Inside that folder make a right click and select "Paste"

In your ObjectDock Bar choose an empty space and right click and select "Dock Settings".

In the ObjectDock Properties windows go to the "Appearance" section, go to the Background roll down and select "Hypergrass".