How to change color of the out of bounds background when not in fullscreen?

So I’m using size(720, 720) instead of fullScreen(). This leaves the ugly gray background. I want to change it to black, kinda like this:

First screenshot’s black zone is the actual canvas.
This “full screen” background does not need to be changed, it’s always black.

I can replicate this with using fullScreen() and translating and clip()ping (or rendering to PGraphics and then image(pseudoCanvas, whatever coordinates are center, 720, 720), but it’s less elegant. But if there’s no way to change the color I will have to resort to that option.

Personally, I would use the fullScreen() approach and center a rectangle into which I would draw the buttons as needed for the various screens.

void setup() {

void draw() {
 rect(150, 400, width - 300, height - 800);