Gnome3 change theme per application

Question

Does anyone know what's application on the screenshot?

Answer 1

This is the "GTK Inspector", a developer tool that's part of GTK 3. (Earlier known as "gtkparasite.") All changes you make within it are temporary and lost upon closing the program.

In most GTK 3 programs you can open it using CtrlShiftI or CtrlShiftD, or by running the program from command line as:

GTK_DEBUG=interactive appname

GTK 3 also lets you override just the theme, using:

GTK_THEME=themename appname