Unlike some of the others, Squarespace is a "closed platform" which means you can't purchase a template from outside of Squarespace.
The way Squarespace Template Detection works is pretty different than any other CMS or website building platform. Squarespace uses a "theme ID" and each ID is correlated to a specific theme / a list of themes. That's right - with some sites we will know exactly what theme is being used, with others we have a list of optional themes. It's kind if strange but it's the only way that works.
When getting the results for any given Squarespace site, this is what you can expect: