Parasite is a debugging and development tool that runs inside your GTK+ application's process. It can inspect your application, giving you detailed information on your UI, such as the hierarchy, X window IDs, widget properties, and more. You can modify properties on the fly in order to experiment with the look of your UI.
https://chipx86.github.io/gtkparasite/
https://chipx86.github.io/gtkparasite/