To create a fully-featured theme that is selectable in the dropdown, you would need to open a pull request and get it incorporated into the codebase. Or, if you are on an instance other than piefed.social, convince your local admin to incorporate your theme into their code.

However, most of the themes you see in piefed are pretty much just a css file that overwrites the default (here is the one for the amethyst theme as an example). AFAIK, only the dillo theme really uses a lot of alternate template files, but that is due to a lot of limitations present in the dillo browser which that theme was designed for.

So, you could pretty much define a new theme using your custom css in the user settings. The only issue might be that some css selectors don't work in the custom css field. Specifically, using the > css selection operator doesn't work (there are probably others too). So, you might run into some roadblocks if you try to get too fancy.