Is there a name for this?

[Willibab]

The background that is slightly darker color, is that called something? Command background? I'm trying to find out if its possible to change/edit them but I don't even know what they are called xD

 

[r66r]

Unless I'm mistaken, you can find the code that generates this backgroud in the rmmz_windows.js:

Window_Selectable.prototype.drawBackgroundRect = function(rect) {
    const c1 = ColorManager.itemBackColor1();
    const c2 = ColorManager.itemBackColor2();
    const x = rect.x;
    const y = rect.y;
    const w = rect.width;
    const h = rect.height;
    this.contentsBack.gradientFillRect(x, y, w, h, c1, c2, true);
    this.contentsBack.strokeRect(x, y, w, h, c1);
};

And the itemBackColor1 and itemBackColor2 are in rmmz_managers.js:

ColorManager.itemBackColor1 = function() {
    return "rgba(32, 32, 32, 0.5)";
};
ColorManager.itemBackColor2 = function() {
    return "rgba(0, 0, 0, 0.5)";
};

So you can modify them with a plugin that overrides these methods.