There are some themes in the ObjectDock library, though not a lot of them - to find those, you would do the following:
Go to the ObjectDock library > click the underlined "Advanced Search Options" just under the Search text box > use check boxes to narrow the search.
Note - admittedly, the search function and organization of the ObjectDock library is sorely lacking. Search will get you in the ball park, but you will still need to surf the pages.
Here is a link to the results of using advanced search and selecting OS themes with no text in search box https://www.wincustomize.com/Skins.aspx?LibID=29&view=1&sortby=5&sortdir=DESC&p=1&advanced=1&scat=OS%20Themes
You can also create your own dock using various backgrounds, dock entries (links), and images - then use the "Save Theme as" function, which will write a configuration (".ini") file. The theme name will now be listed in the theme list (this is the ".ini" file you are seeing, which is now located inside of the "C:\Program Files\Stardock\ObjectDock\Themes" folder).
Downloading theme files can be a bit confusing depending on how the author packaged the theme. Ideally, you would receive a ".zip" folder with all contents, or a ".odtheme" file which you double-click and it all installs to the proper location automatically.
In the case of the ".odtheme" file, it will install to the locations where they were residing on the authors machine, so if the file structure was a bit different - things may go sideways.
With a ".zip" file, you would extract the files to the location you downloaded to, and then move the contents to the proper locations inside of the "C:\Program Files\Stardock\ObjectDock" folder.
In general, you use the following guidelines:
- Background folder bearing the theme name goes in "C:\Program Files\Stardock\ObjectDock\Backgrounds" folder.
- Theme ".ini" file goes in the "C:\Program Files\Stardock\ObjectDock\Themes" folder.
- Any docklets included (that are not already part of the ObjectDock program) would go into the "C:\Program Files\Stardock\ObjectDock\Docklets" folder.
- Folder with the theme images (".ico", or ".png" files) goes in either the "C:\Program Files\Stardock\ObjectDock\Images" folder, or in the "C:\Program Files\Stardock\ObjectDock" folder - depending on how you have your file structure set up.
Note - I use multiple folders for my images inside the "C:\Program Files\Stardock\ObjectDock" folder, which makes it easier to locate images when using the "Change Image" button.
If you keep all your image files and/or folders in the "C:\Program Files\Stardock\ObjectDock\Images" folder - the images will be bundled together, and you end up with a lot of images to sort through when looking to change the image.
The link Bichur gave you above is to my mini tutorial which may better illustrate the file structure issue.
In summary:
You add more themes to your theme list by either saving a theme, or by placing a downloaded theme ".ini" file in the "C:\Program Files\Stardock\ObjectDock\Themes" folder.
You need to place the files from a downloaded ".zip" file in the proper locations in "C:\Program Files\Stardock\ObjectDock".
If you download a theme and end up with a ".odtheme" file, you can either double-click it to install it (and hope all goes well), or you can change the extension to a ".zip" file > extract to same location > manually place the files (this is the method I use).
I hope this helps.