Basically, I am checking to see if I am missing something here. For instance, I want to add the ability for users to create custom themes (and eventually, addons) for my game. In Steam, those would be distributed via the steam workshop. Is there something similar here on itch? If not, for those of you who have tackled this problem.. how did you handle it?

The Itch client is not the same as the Steam client. Including the environment around it. Games are not managed by Itch. Many cannot even be installed by the Itch app.

There is no support for DLC or any kind of sub-projects.

Purchasing a project gives access to the files that are paid only on the project page. One can build a manifest file for the Itch app or hope that the magic of it will handle the installation. But basically Itch is a file hoster with attached store and a browsing cataglogue.

So, no, there is no such thing as modding support or whatever you can do with the Workshop.

However. If you do have a modding community, those mods can be released on Itch as projects by your modders. And you can link to them however you like. Be it a collection or links on your project page, or whatever.

Itch even has a separate category for game mods. https://itch.io/game-mods Maybe you can find examples there. There is bound to be mods for games that are hosted on Itch too.