To see it in action:
Crust.piefed.social (the Piefed development instance)
Not all instances implement it yet, I guess it depends how often they pull from the dev branch
Example of search for 'movies'
I don't think so, here is the list of them (accessible for someone without a PieFed account, although for someone with an account it is under Settings -> Theme, change the dropdown then click Save settings, currently no way that I know of to test one out before saving): https://codeberg.org/rimu/pyfedi/src/branch/main/app/templates/themes