It's the Materia-light Window Manager theme modified to make the active windows "blue" and thus more distinct from the inactive ("gray"). Just like what you would find in Windows, if you will. I literally just copied the Materia-light folder and painted the .svg files, I also made the text in active titles white. Inactive windows were not modified.

I am completely new to Linux and all this, so please let me know if something works not as intended, or if something could be more improved.