i've downloaded some bg from wincustomize, unzipped them into OD folder, but i cant use them! when i opened OD settings, the downloaded themes do not appear, they're just blank like this:
I have a number of blank space like that too, but I don't thing that's related to the problem.
I installed some backgrounds which went to %username% > Documents > Stardock > Object Dock Library > Backgrounds. All I did was move them to C:\ Program Files > Stardock > Object Dock > Backgrounds > non tabbed, and they appeared OK.
It could be to do with files not installing in the same place they used to.
If you place them in C:\ Program Files > Stardock > Object Dock > Backgrounds it won't matter about the tabbed/non-tabbed. Depending on the backgrounds you downloaded, some of mine for instance, you won't see the preview in your config screen like you posted but you would see the names.
Try moving them to the folder I listed (make sure they are in their own folder inside that Backgrounds folder). Close the OD config screen and then re-open it, they should show up.