Owned by Simularweb; an analytic user tracking company. Sends "anonymous" data unless you opt out. They're not shy about using the addon and website for "conducting business"; marketing analysis, and aggregated data analysis with owned affiliates. The data is kept indefinitely. You know the saying "If it's free, you're the product"

The data sent back to mothership if you dont opt out includes
* Installed user styles count
* Visited web pages
* Installed styles for visited webpages

You can opt out by unticking "Send anonymous data..."

https://www.bleepingcomputer.com/news/software/2-million-users-impacted-by-new-data-collection-policy-in-stylish-browser-add-on/

They own stylish.org; the central hub for user styles, which is pretty much unavoidable.

I've been using this addon for years and I feel cheated. The new behavior doesn't really affect the addon's usefulness but It's the principle of it. I never signed up for this. The current owner Justin Hindman is a sellout... if you care about your privacy. Uninstall or use version 2.0.7 (2016) that hasn't been tainted with tracking nonsense.

Firefox 57+ users I highly recommend using Stylus instead (https://addons.mozilla.org/en-US/firefox/addon/styl-us/). Its functionally similarly and its privacy policy is simple: they don't track you and they don't find you that interesting.

Another one bites the dust.