Is there any way — perhaps I’m just being thick — to set a Theme as unusable by any more sites? To toggle a switch so that the sites that have it don’t have to change, but no new sites may use it? Conceptually, one way to do this would be to hide it from the Theme choice page, so if there’s a way to do that, I’m open as well.
I don’t think we have any settings or anything for doing that.
What kind of use case are you thinking of this for?
User error! An oversight on my part resulted in a theme getting into the production instance that shouldn’t have, but the oversight was also not knowing that this happened, so a site started using it. I can’t/shouldn’t just kick them out of the theme tout court, but I don’t want anyone else to use it.
For my penance, I’ll settle for watching any new site creations to make sure they don’t use the theme, and work harder at getting the existing site off it.
At the same time, I could imagine a circumstance where an organization rolls out a new default theme for sites but doesn’t migrate everyone off the old theme immediately. If they, like us, give site creators a good deal of independence, they might want to shut off the old theme to new sites but not kick existing sites off. Migration from old sites could be complicated by CSS Editor changes, for instance.