1. Make yourself a copy of the theme that has the dock that you like.
2. Leave the dock in the folder, but remove everything else.
3. Rename the folder to "Glass Orb Dock.theme," or whatever name you like.
4. Move the new theme to the top of your Winterboard theme list on your device. It will now load before anything else does in Winterboard, and over-ride anything else that is part of another theme that is lower in the hierarchy.
There are other ways to do this, but I prefer this method, as it gives you the option to change it again later, on the fly, and use it with any other theme that you choose, at any time that you choose.