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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