How to disable letterboxing/pillarboxing in Godot?

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:bust_in_silhouette: Asked By kenoma

Stretch Aspect = Keep: Keep aspect ratio when stretching the screen. This means that the viewport retains its original size regardless of the screen resolution, and black bars will be added to the top/bottom of the screen (“letterboxing”) or the sides (“pillarboxing”).

This stretch aspect is the best one that I believe I want to use since it perfectly positions my game in the middle. However, I don’t like the black bars that show. How do I make it somehow “transparent”, so I can implement some sort of additional backgrounds?