I think forcing people to make their own gui is the way to go.
Id imagine that when you install an os it gives you a text on a black background(so there is no "default" gui that you see for the first time), it asks you what you want for your background(you can import it from a website), then it asks you what style are the windows(you can get them from the web or make them yourself), where are the buttons and how do they look, etc.
This way the user will make the gui and it will be the way they like it. Its important to not give the user a "default" gui because it will implant a gui in their head which will mess with their decisions. Its also important to make the gui creation as user friendly as possible(no messing with configuration files). Another thing that is important is how the user makes the gui. Everyone can agree that dragging and dropping assets from a toolbar is the best option; its intuitive.
If everyone took 20 minutes of their time making their gui, they wont have problems using it.