Stylish is a browser extension that allows users to superimpose custom CSS styles on top of websites and create custom skins. The extension will share anonymized user data with SimilarWeb, a digital market intelligence company. Unchecking this option in the extension’s settings panel will stop the data collection system, but will also break the “Suggested Web Styles”” feature and won’t include the user’s installation in the total “”install count”” The extension’s owner Justin Hindman has provided an opt-out option.”
Source: https://www.bleepingcomputer.com/news/software/2-million-users-impacted-by-new-data-collection-policy-in-stylish-browser-add-on/