WordPress Theme Checker is a Chrome extension that provides extra information about the websites you visit. It checks if they are built using the WordPress platform, and from that, it will display the word "WP" when you visit a site. It also allows you to see the themes used by that site, which you can do by clicking on the icon next to the address bar of your browser.

You can install the extension without worrying about what it can do. All it does is work as a message sender for you. It checks the websites you visit and sends you a notification whenever a site is built with the WordPress platform. The extra information it displays is the name of the theme being used and the version of WordPress being used by that site.