Thank you! Really glad you like it :)
CC0 is simple enough, at the most basic level you can just declare the work to be CC0 within the page description, although you can also use the Creative Commons website to generate a licence notice if you so choose.
Even better is including the licence terms in the metadata for the software - for instance, in my fonts I'll add CC0 to the licence metadata field - this would depend on the nature of the software of course.
Itch also lets you provide the licence for assets and code in page metadata under the Release section, which has the added benefit of helping boost your visibility to people searching for CC0 assets.
Best of luck with the concept! :)